CN110180586B - Alkali metal ion modified titanium silicalite TS-1 for propylene and hydrogen peroxide gas phase epoxidation reaction and preparation method thereof - Google Patents

Abstract

An alkali metal ion modified titanium silicalite molecular sieve TS-1 for propylene and hydrogen peroxide gas phase epoxidation reaction and a preparation method thereof, belonging to the technical field of petrochemical industry. The method comprises the following steps: first, a formulation containing a small amount of TPA⁺An ionic alkali metal hydroxide modification solution; secondly, with a small amount of TPA⁺Carrying out controlled hydrothermal treatment on the TS-1 molecular sieve parent substance by using an ionic alkali metal hydroxide solution; and thirdly, carrying out post-treatment on the hydrothermally modified TS-1 molecular sieve. In the washing process, the wet modified TS-1 molecular sieve is washed by using a low-concentration alkali metal hydroxide solution, so that alkali metal ions are retained on silicon hydroxyl groups of the modified titanium silicalite molecular sieve, and the prepared alkali metal ion modified titanium silicalite molecular sieve has remarkable catalytic performance in the phase epoxidation reaction of propylene and hydrogen peroxide. Introducing a small amount of TPA into the hydrothermal modification solution⁺The ion solves the applicability problem of the parent body of the nanometer TS-1 molecular sieve, and simultaneously reduces the modification loss of the titanium silicalite molecular sieve.

Description

Alkali metal ion modified titanium silicalite TS-1 for propylene and hydrogen peroxide gas phase epoxidation reaction and preparation method thereof

Technical Field

The invention belongs to the technical field of petrochemical industry, and relates to an alkali metal ion modified titanium silicalite TS-1 suitable for propylene and hydrogen peroxide gas phase epoxidation reaction and a preparation method thereof.

Background

Titanium silicalite is a silicate zeolite with a titanium heteroatom in the crystal framework. TS-1 is a very important member of the titanium silicalite family. MarcoTalamasso et al first reported a synthetic method for TS-1 (GB2071071A, USP4410501,1983). It has MFI topological structure and ten-membered ring cross channel system as common Si-Al molecular sieve ZSM-5.

A great deal of basic research shows that the titanium heteroatom is arranged on the TS-1 molecular sieve frameworkIsolated four coordinate forms exist. The characteristic absorption of the transition from oxygen ligand to titanium central atom electron appears near 210nm of ultraviolet visible diffuse reflection spectrum, and the characteristic absorption is 1120cm of ultraviolet Raman spectrum^-1Characteristic resonance absorption occurs nearby. In addition, the skeleton titanium also has a mid-infrared region of 960cm in the infrared spectrum^-1Characteristic absorption belonging to Si-O-Ti antisymmetric stretching vibration (or Si-O bond stretching vibration disturbed by framework titanium) appears nearby. The microenvironment of the titanium scaffolding can vary. According to the report of the publication J.Catal.,1995,151,77-86, the TS-1 molecular sieve was subjected to sodium exchange with 1M NaOH solution at 25 ℃ and then was subjected to sodium exchange at 960cm^-1The infrared characteristic peak of nearby framework titanium disappears, and meanwhile, the peak is 985cm^-1Where a new characteristic absorption of the infrared occurs. Some literature suggests that the sodium exchange that occurs in such strongly alkaline solutions is essentially a reaction of sodium hydroxide with silicon hydroxyl groups near the backbone titanium (NaOH + Si-OH → Si-O)^-Na⁺+H₂O), consequently, the microenvironment of the framework titanium is changed, and the infrared spectrum characteristics of the framework titanium are influenced.

A large number of application researches show that the TS-1 molecular sieve obtained by introducing titanium heteroatom into MFI framework has unique catalytic oxidation performance and wide application. In brief, the TS-1 molecular sieve can catalyze epoxidation of a series of olefins with low concentrations of hydrogen peroxide to form epoxides. Besides, the TS-1 molecular sieve can be used as a catalyst for phenol hydroxylation, cyclohexanone ammoxidation and alcohol oxidation to generate aldehyde, ketone and alkane oxidation to generate alcohol, ketone and the like. Neri et al first reported a propylene liquid phase epoxidation process using methanol as solvent and 30 wt.% hydrogen peroxide as oxidant (USP 4833260, 1989) and obtained results with hydrogen peroxide conversion and Propylene Oxide (PO) selectivity of greater than 90%. Clerici et al have studied the reaction law of various lower olefins with hydrogen peroxide catalyzed by TS-1 molecular sieves (J Catal, 1993, 140 (1): 71) and have shown that the order of the reaction rates for the liquid phase epoxidation of olefins in various solvents is: methanol > ethanol > tert-butanol. In 2008, Degussa and Uhde, and BASF and Dow, respectively, combined a process of propylene liquid phase epoxidation (HPPO) based on a TS-1 titanium silicalite molecular sieve and a methanol solvent with a process of producing hydrogen peroxide by an anthraquinone method, led to a green plant for producing propylene oxide (ind.

Although TS-1 titanium silicalite molecular sieves synthesized by different methods have olefin epoxidation catalytic performance, practical TS-1 catalysts of propylene liquid phase epoxidation (HPPO) processes are almost prepared by adopting nano TS-1 molecular sieves with aggregate particle size of 200-300 nm as precursors. This molecular sieve is generally synthesized using high purity tetrapropylammonium hydroxide (TPAOH) as a template and according to the hydrothermal synthesis (classical method) described by Taramasso et al (USP4410501.1983) and Thangaraj et al (J Chem Soc Chem Commun, 1992: 123). In order to achieve better reaction effect, many documents also adopt an aqueous solution containing a low-concentration TPAOH template agent to carry out secondary hydrothermal treatment on the nano TS-1 molecular sieve matrix.

The advantages of the nano TS-1 molecular sieve synthesized by the classical method in the propylene liquid phase epoxidation (HPPO) process include: the non-skeleton titanium is less, which is beneficial to reducing the decomposition of the non-skeleton titanium on the hydrogen peroxide; the grain size is small, the channel is short, and the resistance effect of the ten-membered ring micropores on the diffusion of reactants and products in the liquid phase reaction is favorably reduced. However, it is well known to those skilled in the art that the classical synthesis of TS-1 molecular sieves requires the use of a high purity templating agent TPAOH and a source of silicon and titanium esters to prevent the introduction of contaminating metal ions, especially sodium and potassium alkali metal ions. It has been found that the presence of even a very small amount of alkali metal ions in the hydrothermal synthesis of TS-1 molecular sieve significantly reduces the amount of titanium incorporated into the molecular sieve framework, thereby significantly reducing the liquid phase oxidation activity of the catalyst. Publication J.Catal.,1995,151,77-86 demonstrates the effect of sodium ion content on TS-1 molecular sieves by adding a sodium salt system to the gel from which the TS-1 molecular sieves were synthesized. The data provided in the document show that when the content of sodium ions in the gel is too high (molar ratio of Na/Si ≧ 0.05), the synthesized TS-1 molecular sieve has no catalytic activity for the oxidation of n-octane; only when the content of sodium ions in the gel is very low (the molar ratio of Na/Si is less than or equal to 0.01), the catalytic activity of the synthesized TS-1 molecular sieve is close to the normal level. The data provided in this document also show that high sodium content, while significantly reducing or even completely losing the catalytic activity of the TS-1 molecular sieve, significantly promotes the decomposition of hydrogen peroxide. Based on the results of the liquid phase oxidation reaction, an empirical value of 0.01 is given as the maximum allowable value of alkali metal ions in the gel during hydrothermal synthesis of the TS-1 molecular sieve (expressed in terms of the molar ratio of alkali metal ions to Si atoms, see J.Catal.,1995,151, 77-86; Stud.Surf.Sci.Catal.,1991,69, 79-92; Stud.Surf.Sci.Catal.,1991,60, 343-352; appl.Catal.A-gen.,2000,200, 125-134; Front.Chem.Sci.Eng.,2014,8, 149-155.). According to the standard, the content of alkali metal ion impurities in the TPAOH solution of many commercial templates exceeds the standard. Therefore, it is proposed to add cation exchange resin to the synthesized gel during hydrothermal synthesis of TS-1 molecular sieve. This is because the cation exchange resin can capture alkali metal cations in the synthetic gel by ion exchange (RSC adv.,2016, DOI:10.1039/C5RA 23871D.).

In the propylene liquid phase epoxidation reaction (low-temperature high-pressure reaction condition) using methanol as a solvent, the restriction of microporous diffusion resistance on the reaction conversion rate is reduced as much as possible, which is a main problem to be solved when preparing the catalyst. Therefore, secondary hydrothermal treatment of TS-1 molecular sieve precursors with aqueous TPAOH templating agents is a common method for modifying TS-1 molecular sieve precursors. In the process, partial dissolution-recrystallization process occurs on the parent TS-1 molecular sieve. The method aims to utilize the partial dissolution-recrystallization process to generate mesopores or/and hollow cavities in the crystal of the TS-1 molecular sieve and communicate with micropores, so as to achieve the purposes of further improving the micropore diffusivity of the nano TS-1 molecular sieve and improving the catalytic activity of the nano TS-1 molecular sieve in the liquid-phase epoxidation reaction.

For example, chinese patent application nos. 98101357.0, 98117503.1, 99126289.1 and 01140182.6 disclose modification methods involving organic bases, wherein aqueous solutions comprising quaternary ammonium bases are used to perform secondary hydrothermal treatment of TS-1 molecular sieve precursors. The published microporouus and mesorouus Materials 102(2007)80-85 researches by using methods such as nitrogen physical adsorption, transmission electron microscopy and diffuse reflection ultraviolet-visible spectroscopy and the like show that a partial dissolution-recrystallization process does occur in the process of TPAOH solution hydrothermal modification of TS-1. The process produces two direct modification effects on the TS-1 molecular sieve: firstly, a cavity is generated in the crystal grain, and secondly, part of non-framework titanium (object titanium species, such as anatase) is converted into framework titanium. The former can be favorable for reaction by shortening the length of micropores and reducing the micropore diffusion resistance of the TS-1 molecular sieve, and the latter is favorable for reaction by removing object species in the micropores and dredging micropore channels, so that the number of active centers is increased and the number of titanium oxide species for decomposing hydrogen peroxide is reduced. In addition, the Chinese patent application No. 201010213605.0 discloses the modification of TS-1 molecular sieve with water vapor of organic alkali. The modification effect is illustrated by the reaction (liquid phase) of phenol hydroxylation to synthesize benzenediol. The invention aims to overcome the defect of large wastewater discharge amount in the organic alkali solution hydrothermal modification method.

In addition to the above-mentioned secondary hydrothermal modification (also called partial dissolution-recrystallization modification) methods aimed at solving the problem of micropore diffusion limitation, the following documents report several modification methods for the titanium active center and its microenvironment of the titanium silicalite molecular sieve.

For example, the publication chem. Eur.J.2012,18, 13854-13860 reports the first characterization of a new hexa-coordinated framework titanium active center by UV Raman spectroscopy (Ti (OSi))₂(OH)₂(H₂O)₂) The research work of (1). Involving the use of low concentrations of NH containing small amounts of hydrogen peroxide₄HF₂The solution is used for processing the TS-1 molecular sieve (80 ℃), and the result is that the isolated four-coordination framework titanium active center (Ti (OSi)) in the conventional TS-1 molecular sieve is treated₄) Conversion to a skeletonlike titanium active site (Ti (OSi)) which is likewise isolated but has a hexacoordination number₂(OH)₂(H₂O)₂). The novel six-coordination framework-like titanium active center is 695cm in ultraviolet Raman spectrum^-1Generates characteristic absorption and shows good catalytic activity in the propylene liquid phase epoxidation reaction. From the spectrum provided in this document, it was confirmed that the modification method described above is carried out by using only a part of the titanium active site having a four-coordinate skeleton (absorption site: 960 cm)^-1Where) is converted into a skeleton-like active center without changing the microenvironment (at 960 cm) of the remaining tetra-coordinated framework titanium^-1The characteristic absorption of (a) is not displaced). In fact, such a radicalIn hydrogen peroxide and NH₄HF₂The solution modification method has been reported in the following documents: angew. chem.2003,115, 5087-5090; Angew.chem.Int.Ed.2003,42, 4937-; adv.synth.cat.2007, 349, 979-986; appl.Catal.A 2007,327, 295-299. The catalyst modified by the method can improve the catalytic activity and selectivity of the benzene hydroxylation reaction (liquid phase).

The publication Phys. chem. Phys.2013, 15, 4930-4938 mentions a process for preparing a catalyst containing NH₄Treating a layered titanium silicalite Ti-MWW (treatment temperature is 40-150 ℃) by using a 2M nitric acid solution of F, and preparing a framework fluorine-containing titanium silicalite (F-Ti-MWW). When fluorine atoms are bonded to silicon adjacent to the titanium skeleton, i.e. SiO appears next to the titanium skeleton_3/2F, the microenvironment of the framework titanium is changed. SiO 2_3/2F increases the electropositivity of the titanium center by electron withdrawing, and the titanium center with increased electropositivity is increased by generating O with stronger electrophilicity^α(Ti-O^α-O^β-H) increases the activity (liquid phase) of the olefin epoxidation reaction. The Chinese patent application No. 201210100532.3 also discloses a preparation method and application of the MWW structure and framework fluorine-containing molecular sieve. In addition, the publication ACS Catal, 2011,1,901 and 907 shows that the NH4F modification can reduce hydrophilic hydroxyl on the surface of TS-1, thereby improving the surface hydrophobicity of the catalyst.

Publication chem. commu.2016, 52,8679 provides a modification of TS-1(170 ℃) by hydrothermal treatment with a solution containing both ethylamine and tetrapropylammonium bromide. The method comprises selectively dissolving silicon and recrystallizing the dissolved silicon to obtain titanium (Ti (OSi))₄) Conversion to the six-coordinate, still backbone-linked, active titanium species (Ti (OSi)₂(OH)₂(H₂O)₂). The method and the aforementioned low-concentration NH containing a small amount of hydrogen peroxide₄HF₂The modification effect of the solution is isogeny. But the method is easy to optimize the proportion of four-coordination active titanium species and six-coordination active titanium species in TS-1, can avoid the problem of titanium removal of fluoride, and has obvious advantages. Moreover, the modified TS-1 molecular sieve is used for cyclohexene epoxidation (liquid phase) and shows modification effects of improving activity, selectivity and the like.

In addition, there are a few reports on methods for ion modification of titanium silicalite molecular sieves.

For example, chinese patent application No. 201480052389.2 discloses a method for preparing a zinc-modified titanium silicalite catalyst, which involves impregnating a TiMWW molecular sieve with an aqueous solution containing zinc acetate (100 ℃), followed by filtering, washing, spray drying, etc. to obtain a ZnTiMWW molecular sieve containing 0.1 to 5 wt.% (1.6 wt.% in the examples). The Zn-containing TiMWW molecular sieve can be used for synthesizing propylene oxide in a liquid-phase propylene epoxidation reaction system, but acetonitrile is used as a solvent.

As another example, the application of Catalysis A in General 200(2000) 125-134 reported the alkali metal ion exchange treatment of TS-1 molecular sieves with potassium carbonate solution at room temperature. As in the publication J.Catal.,1995,151,77-86, the sodium exchange of TS-1 with 1M NaOH solution at 25 deg.C, potassium ions in the strongly basic potassium carbonate solution can also exchange with the nearby silicon hydroxyl groups of the framework titanium, resulting in replacement of hydrogen ions on the silicon hydroxyl groups, thereby changing the microenvironment of the framework titanium and affecting the infrared spectral characteristics of the framework titanium. However, from the results of the liquid phase oxidation reaction of hexane and 2-hexene provided, the alkali metal ion exchange treatment at room temperature reduces the activity of the TS-1 molecular sieve for the liquid phase oxidation reaction of hexane and 2-hexene.

Since it has been known in the synthesis research of TS-1 molecular sieves that the presence of alkali metal ions is not favorable for titanium entering the framework in the hydrothermal synthesis of TS-1, and the introduction of alkali metal ions into TS-1 molecular sieves after synthesis has been found to be also unfavorable for the liquid-phase oxidation reaction using hydrogen peroxide as an oxidant through ion exchange research, it has not been known to date what catalytic applications alkali metal ion modified TS-1 molecular sieves have.

However, some publications such as Catal.Lett.,8,237(1991) and stud.Surf.Sci.Catal.,84,1853(1994) report that TS-1 molecular sieve frameworks often contain very low levels of trivalent metal ion impurities (e.g., Al)³⁺、Fe³⁺) They produce bridged hydroxyl groups with strong protonic acidity. This very small amount of strongly acidic centers will cause TS-1The liquid phase oxidation reaction product obtained by the catalysis of the molecular sieve further generates acid catalysis side reaction, and the selectivity of the reaction is reduced. The extremely low content of alkali metal ions introduced into the TS-1 molecular sieve can effectively avoid the damage of acid centers to the selectivity of the catalyst. But in this case the very low content of alkali metal ions acts as a counter cation to neutralize the acid sites. For liquid phase oxidation reactions, the introduction of alkali metal ions into the TS-1 molecular sieve, once in excess of the amount necessary to neutralize the acid sites, can cause side effects such as a decrease in catalyst activity.

In addition, it is known to those skilled in the art that various low-temperature selective oxidation reactions catalyzed by titanium silicalite molecular sieves use aqueous hydrogen peroxide as an oxidant. Commercial hydrogen peroxide solutions often contain 200-300 ppm of an acidic stabilizer (50 wt.% H)₂O₂The pH of the reaction solution is about 1-2), the acidic stabilizer enters a titanium silicalite molecular sieve catalytic reaction system along with hydrogen peroxide, and the reaction medium is acidified (in a propylene epoxidation reaction medium, hydrogen peroxide-methanol feed (3 molH)₂O₂a/L) of about 3.0), which likewise reduces the reaction selectivity. In addition, hydrogen peroxide molecules can generate transient peroxyprotons (Ti (eta) with strong acidity when activated by a five-membered ring mode on the titanium active center of the titanium-silicon molecular sieve²)-O-O^-H⁺). To counteract the effect of these acidity on the selectivity of propylene epoxidation reactions, many patents have resorted to the addition of a basic material to the reaction medium. Basic additives as mentioned in the chinese invention patent (application No.) 201410512811.x are ammonia, amines, quaternary ammonium bases and M¹(OH)_n，M¹Is an alkali metal or an alkaline earth metal; the additives mentioned in chinese invention patent (application no) 00124315.2 are alkali metal hydroxides, alkali metal carbonates and bicarbonates, alkali metal carboxylates and ammonia; chinese invention patent (application no) 03823414.9 mentions the addition of less than 100wppm of alkali metal, alkaline earth metal, alkali or alkali cation having a pKB of less than 4.5, or combinations thereof to the reaction medium. Wherein wppm is based on the total weight of hydrogen peroxide in the reaction mixture; the invention of Chinese patent (application number) 201180067043.6 is reverse110 to 190 micromoles of potassium cations and at least one phosphorus in the form of a hydroxy acid anion are added to the medium. The micromolar addition is based on 1 mole of hydrogen peroxide in the feed. Thus, the literature teaches that the non-inherent acidity of the catalyst is addressed by adding to the reaction medium or mixture an alkaline material comprising an alkali metal hydroxide or a weak acid salt which hydrolyzes to form hydroxide. The range of the amount of the alkaline substance to be added is generally determined based on the amount of the hydrogen peroxide in the feed. According to the information disclosed in chinese patent application No. 201480052389.2, at least a major portion of the alkaline material added to the reaction medium flows out of the reactor outlet with the reaction product.

However, the application of catalyst A in General 218(2001) 31-38 teaches that in the liquid phase epoxidation of propylene in a batch tank, the addition of a small amount of sodium carbonate to the reactants, which is intended to increase the pH of the reaction solution and to inhibit further side reactions of the epoxidation product with the solvent, thereby increasing the selectivity for propylene oxide, tends to cause deactivation of the catalyst due to the accumulation of sodium carbonate thereon. The results in Table 5 in this document show that Na is added to the solution when sodium carbonate is impregnated on a TS-1 molecular sieve₂The loading of O reached 1.36 wt.% (corresponding to about Na/Si ═ 0.027), the activity of the catalyst (hydrogen peroxide conversion) decreased by nearly half.

It is noted that the TS-1 molecular sieve is made to contain very low levels of alkali metal ions to replace very low levels of trivalent metal ion impurities (e.g., Al) on the molecular sieve framework³⁺、Fe³⁺) The generated bridged hydroxyl (protonic acid) avoids the oxidation product from further acid catalysis side reaction, thereby improving the selectivity of the catalyst; and adding a small amount of alkaline substance to the reaction medium, wherein the addition of alkali metal ions or hydroxides thereof is included, so as to neutralize the acidity of the reaction medium and transient peroxyprotons generated by the hydrogen peroxide activated by the TS-1 molecular sieve, are not modified by the alkali metal ions.

In addition, we note that the hydrothermal modification of TS-1 molecular sieves with aqueous solutions of mixtures of organic and inorganic bases is disclosed in Chinese patent application No. 200910131992.0And (4) a sexual method. Wherein the inorganic base relates to ammonia water, sodium hydroxide, potassium hydroxide and barium hydroxide; the organic bases are urea, quaternary ammonium bases, fatty amines and alcohol amine compounds. Examples 2, 3,4, 7, 8 and 11 of this patent relate to the use of sodium hydroxide and ethylenediamine, potassium hydroxide and TPAOH, potassium hydroxide and triethanolamine, sodium hydroxide and n-butylamine, potassium hydroxide and TPAOH, respectively, and a base combination of sodium hydroxide and TPAOH. The temperatures for the hydrothermal modification in the above examples were 180 deg.C, 150 deg.C, 180 deg.C, 120 deg.C, 90 deg.C and 180 deg.C, respectively. The invention uses phenol hydroxylation to synthesize benzenediol reaction (liquid phase) and cyclohexanone ammoxidation reaction (liquid phase) to illustrate the overall improvement of the modified molecular sieve in the aspects of activity, selectivity and activity stability. It is noted that the invention uses Fourier transform infrared spectroscopy (FT-IR) to confirm that modified TS-1 molecular sieves, including TS-1 molecular sieves involving inorganic bases in modification, have framework titanium with an infrared absorption band of 960cm as the unmodified matrix^-1To (3). Therefore, the patent uses 960cm^-1Intensity of absorption band (I) of₉₆₀) And 550cm^-1Intensity of absorption band (I)₅₅₀) The ratio characterizes the effect of mixed base modification on the relative content of framework titanium. The Chinese patent application No. 200910131993.5 discloses a method for carrying out hydrothermal modification on a TS-1 molecular sieve by using inorganic alkali and/or organic alkali aqueous solution containing pore-forming agent. Wherein, the inorganic base relates to ammonia water, sodium hydroxide, potassium hydroxide and barium hydroxide. Examples 2 and 3 relate to a sodium hydroxide modified solution containing starch and a potassium hydroxide modified solution containing polypropylene, respectively. Similar to the Chinese patent application No. 200910131992.0, the invention also uses the reaction (liquid phase) of phenol hydroxylation to synthesize benzenediol and the reaction (liquid phase) of cyclohexanone ammoxidation to show the overall improvement of the modified molecular sieve in terms of activity, selectivity and activity stability. Furthermore, Fourier transform infrared spectroscopy (FT-IR) was also used to confirm that the modified TS-1 molecular sieves, including TS-1 molecular sieves modified with inorganic bases, have an infrared absorption band of 960cm for titanium as the matrix^-1To (3). Therefore, the patent also uses 960cm^-1Intensity of absorption band (I) of₉₆₀) And 550cm^-1Intensity of absorption band (I)₅₅₀) The ratio characterizes the effect of the alkali modification on the relative content of framework titanium. The above invention does not mention whether the modified TS-1 contains alkali metal ions. The invention aims to improve the micropore diffusion performance of the TS-1 molecular sieve by hydrothermal modification. However, the Fourier transform infrared spectroscopy (FT-IR) test results provided by the inventions are that the modified TS-1 molecular sieve comprises the TS-1 molecular sieve which involves the modification of inorganic base, and the infrared absorption band of the framework titanium is still 960cm^-1Is an important feature. It is shown that, by the modification method provided by the above invention, the antisymmetric stretching vibration of Si-O-Ti bond (or Si-O bond stretching vibration disturbed by the framework titanium) is not affected by alkali metal ions. In other words, the alkali metal ions do not affect and alter the microenvironment of the framework titanium. The most reasonable explanation is that the invention removes most of the alkali metal ions possibly contained in the modified TS-1 molecular sieve by eluting through the post-treatment steps such as water washing which are commonly used.

In addition, we have noted that the following patents relate to lye treatment. However, these so-called lye treatments are also not alkali metal ion modifications to be provided by the present invention.

For example, the TS-1 shaping method disclosed in Chinese patent application No. 201010511572.8 involves a lye treatment step, and the catalyst prepared by the method is used for the liquid-phase epoxidation of propylene to produce propylene oxide. However, the patent is characterized in that a hollow TS-1 molecular sieve (which is subjected to secondary hydrothermal modification by a modification method referred to Chinese invention CN1132699C and application No. 99126289.1) is firstly molded by taking silica sol containing silane and/or siloxane with at least two hydrolyzable groups as a binder to obtain a molded body. The shaped bodies are then heat treated with an alkaline solution involving sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide and tetraethylammonium hydroxide, dried and calcined to give a TS-1 catalyst with sufficient crush strength and ultra-high molecular sieve content. Wherein the heat treatment temperature is 60-120 ℃, the concentration of the alkali liquor is 0.1-10 mol%, and the ratio of the alkali liquor to the formed body is (0.5-5)/1. It can be clarified that the lye heat treatment referred to in the present invention is not intended to modify the shaped bodies but to promote the hydrolysis reaction of the silanes and/or siloxanes in the binder in order to obtain a sufficient crushing strength of the shaped bodies. This can be confirmed by the statements in paragraph [0034] ("the amount of base used can be selected according to the amount of silane and/or siloxane having at least two hydrolyzable groups"). Further, according to comparative analysis of the heat treatment temperature, time and index data of examples 1 to 7, i.e., the particle strength, the hydrogen peroxide conversion rate and the propylene oxide selectivity, the upper limit of the heat treatment temperature of 120 ℃ corresponds to the lower limit of the treatment time of 2 hours. That is, in order to achieve good molding effect, the temperature of the alkali solution heat treatment is not high, and the time is not long, otherwise the reactivity and selectivity of the liquid phase epoxidation of propylene are reduced. Among them, the conditions of the heat treatment with the alkali solution at 90 ℃ for 6 hours in example 4 were the optimum values.

For another example, the preparation method of the high performance titanium silicalite molecular sieve catalyst disclosed in the chinese patent application No. 201310146822.6 also involves modification treatment of micron-sized titanium silicalite molecular sieve with alkaline solution. The preparation method is mainly characterized in that the micron-sized titanium silicalite molecular sieve catalyst is treated by alkaline solution, so that a large number of mesopores and macropores are formed on the micron-sized titanium silicalite molecular sieve, the accessibility of titanium active centers in the large-grain titanium silicalite molecular sieve is improved, and the product molecules are favorably diffused and guided out from the inside of the large grains. Then, the titanium-containing modified liquid is used for treating the titanium-silicon molecular sieve catalyst containing mesopores and macropores again, and the aim is to introduce more active centers to the surface of the molecular sieve catalyst through a crystallization process. It can be clarified that although alkali metal hydroxide is mentioned in the alkaline solution treatment step of the invention, the patent requires that the molecular sieve treated by the step should satisfy the requirement of neutral pH value by washing with water. This means that the water wash in this step must be very sufficient. In this way, even if the alkali metal hydroxide is used to complete the modification by the alkali solution treatment, no alkali metal ion remains on the catalyst. Because there is residual alkali metal ion, there will be residual hydroxide anion, which can not satisfy the strict requirement of pH value being neutral. The invention therefore places such high demands on this water washing in connection with the modification of the molecular sieve surface to be introduced with skeletal titanium active centers in a subsequent second step. It will be appreciated from the foregoing background description that if a large amount of alkali metal ions were allowed to be contained in the molecular sieve in the first alkaline solution treatment step of the invention, it would be inevitable to prevent the titanium from efficiently entering the framework to become active sites in the second modification step.

In summary, in the development of an environmentally friendly oxidation process using a green oxidant, hydrogen peroxide, the preparation method of the titanium silicalite molecular sieve catalyst is mostly studied according to a liquid phase reaction mode in the prior patents and publications. Generally, the liquid phase oxidation reaction is carried out at low temperature. Under such conditions, the hydrogen peroxide self-decomposition reaction is slow, and therefore the selectivity (effective utilization rate) of hydrogen peroxide for selective oxidation reaction in the liquid-phase reaction mode is high. The challenge in developing titanium silicalite catalysts suitable for the liquid phase reaction mode is mainly how to improve the low temperature reactivity of the catalyst. The mass transfer diffusion resistance of micropores of the titanium-silicon molecular sieve under the liquid phase reaction condition is a large factor for limiting the low-temperature activity of the catalyst. This is why many patents and publications are dedicated to the preparation of catalyst by using the nano TS-1 molecular sieve synthesized by the classical method as a precursor, and performing secondary hydrothermal recrystallization treatment (usually in the presence of tetrapropylammonium hydroxide) on the precursor as much as possible in order to generate appropriate mesopores and cavities inside the ultrafine TS-1 molecular sieve grains, thereby providing good catalytic effect for the catalyst.

Different from the prior invention, the alkali metal ion modified catalyst and the preparation method thereof provided by the invention are specially used for the gas-phase epoxidation reaction of propylene and hydrogen peroxide. The propylene gas phase epoxidation reaction is carried out under the conditions of normal pressure and temperature higher than 100 ℃ without the participation of a solvent. Under these conditions, the reactants propylene and hydrogen peroxide are directly contacted in the form of gas molecules, and pass through the catalyst bed together to perform epoxidation reaction. It is understood that the reaction mechanism of the propylene gas phase epoxidation cannot be equal to that of the liquid phase epoxidation due to the change of the reaction phase and conditions and the absence of the solvent methanol, the required active center is different from that of the liquid phase epoxidation, and the main challenges and the main problems to be solved by the preparation and modification of the catalyst are different from those of the liquid phase epoxidation.

But some are very specific. That is, the gas phase epoxidation of propene and hydrogen peroxide has a great potential advantage over the known liquid phase epoxidation processes. This is a significant value of the present invention. It is known that the liquid phase epoxidation process (HPPO) of propene requires the use of large amounts of solvent to ensure that propene (oil) and aqueous hydrogen peroxide can be mixed through liquid-liquid mixing into a stable homogeneous phase in order to safely carry out the epoxidation reaction. Currently, the production units of the HPPO process all use methanol as solvent. Although the methanol is widely available and cheap, and the methanol solvent is considered to have the possibility of forming a so-called five-membered ring transition state with hydrogen peroxide molecules and framework titanium active centers and additionally has the effect of promoting hydrogen peroxide activation and epoxidation, the methanol solvent also brings great troubles to the practical application of the HPPO process. Firstly, methanol is easy to generate solvolysis side reaction with a propylene oxide product to generate byproducts such as high-boiling-point substance propylene glycol monomethyl ether and the like. These by-products not only severely reduce the selectivity of propylene oxide, but also increase the difficulty of wastewater treatment. Secondly, the methanol solvent must be recycled and needs to undergo complicated refining treatment (including hydrogenation, rectification and resin adsorption) before being recycled, which leads to complicated HPPO process flow and high investment and energy consumption. In addition, despite the complicated purification of the recycled methanol solvent, more than ten and even two to thirty trace impurities (including fusel alcohols, aldehydes, ethers, esters and oxacycles) are difficult to remove. The trace impurities return to the reactor along with the circulating methanol, so that the inactivation of the catalyst is accelerated, and the service life of the catalyst is seriously shortened. The gas phase epoxidation reaction of propylene and hydrogen peroxide does not use any solvent, so the problems are thoroughly avoided, and the method has good development potential.

Since 2002, we have been working on the study relating to the epoxidation of propylene with hydrogen peroxide, first of all, the development of a catalyst capable of reacting with hydrogen and oxygenThe dielectric barrier discharge plasma technology for the direct synthesis of high purity gaseous hydrogen peroxide from mixtures is described in the following documents: chem. Commun.,2005, 1631-; modern chemical industry, vol.26 supplement, 2006, P194-197; AIChE J,53: 3204-; new electrical and energy technology, Vol28,2009, No.3, P73-76; chi.j.cat., 2010,31: 1195-; the chemical bulletin Vol63,2012, No.11, P3513-3518; journal of Catalysis 288(2012) 1-7; angew.chem.int.ed.2013,52, 8446-; AIChE J,64: 981-; chinese invention patent (application number) 200310105210.9,200310105211.3,200310105212.8. We have realized the in-situ continuous synthesis of hydrogen peroxide gas by using the plasma technology and completed the first stage of research work on propylene gas phase epoxidation in 2007 (Zhou Jun Cheng]Dalian university of Dalian graduate, 2007). In particular, the research work employed a specially designed two-stage integrated reactor. The first stage reactor is a Dielectric Barrier Discharge (DBD) plasma reactor for providing a continuous, stable gaseous hydrogen peroxide feed to the epoxidation reaction stage starting with a mixture of hydrogen and oxygen (the concentration of oxygen in hydrogen is less than 6 v%). The second stage reactor is a gas phase epoxidation reactor of propylene and hydrogen peroxide gas, and TS-1 molecular sieve particles are filled in the reactor. The results of the gas phase epoxidation reaction at 90 ℃ and 1atm obtained in this study are: about 7% propylene conversion, 93% Propylene Oxide (PO) selectivity and 0.24kg PO kg TS-1^-1h^-1Propylene oxide yield. Later, we carried out more comprehensive research work using the same system and a non-classical (also inexpensive) method for the synthesis of micron-sized TS-1 (unmodified) as catalyst. The results published in the publication Chin.J.Catal.,2010,31: 1195-1199 show that the selectivity of the propylene oxide can still reach about 95 percent at the reaction temperature of 110 ℃, and the yield of the propylene oxide is kept at 0.25kgPO kgTS-1^-1h^-1On the other hand, the catalyst activity was stable in the continuous 36-hour gas-phase epoxidation reaction. But the epoxidation selectivity, i.e. the effective utilization rate of hydrogen peroxide, is only about 36%.

We have already notedTo date, Klemm et al also reported 2008 work on the gas phase epoxidation of propylene [ Ind. Eng. chem. Res.2008,47,2086-]. They used a special glass vaporizer or a microchannel falling film evaporator and 50 wt% aqueous hydrogen peroxide to provide a gaseous hydrogen peroxide feed for the vapor phase epoxidation reaction, which was a microchannel reactor internally coated with a TS-1 molecular sieve. The reaction results obtained at 140 ℃ and 1atm are: propylene oxide selectivity>90% yield of propylene oxide>1kgPO kg_TS-1 ^-1h^-1. But the effective utilization rate of the hydrogen peroxide is only about 25 percent.

The preliminary research work on propylene gas-phase epoxidation can show that under the condition of no participation of a methanol solvent, propylene is directly contacted with hydrogen peroxide gas, the epoxidation reaction can effectively occur on a TS-1 molecular sieve, the considerable yield of propylene oxide is achieved, and the selectivity of the propylene oxide reaches about 90 percent, which is close to the liquid-phase epoxidation result. However, the effective utilization rate of hydrogen peroxide in the gas-phase epoxidation reaction is very low under the normal feeding proportion of propylene and hydrogen peroxide, and is usually only 20-40%. This value is much lower than liquid phase epoxidation (typically between 85 and 95%). Research studies by Susan et al (Journal of Catalysis 288(2012) 1-7) and Ferrandez et al [ Ind. Eng. chem. Res.2013,52,10126-10132] show that the effective utilization rate of hydrogen peroxide in gas phase epoxidation reaction is very low because the self-decomposition side reaction of hydrogen peroxide at high temperature (e.g. 110-140 ℃) is in strong competition with the main epoxidation reaction. The decomposition reaction of hydrogen peroxide (to form water and oxygen) can occur both on the material surface of the reactor and on the catalyst.

It is clear that the challenge to be met by the process for the gas-phase epoxidation of propene is above all the serious problem of self-decomposition of hydrogen peroxide at high temperatures. The rapid decomposition of hydrogen peroxide at high temperature not only reduces the effective utilization rate of hydrogen peroxide and the conversion rate of propylene, but also generates oxygen which easily causes organic gases in a reaction system and a downstream separation system to enter an explosion limit to cause safety accidents.

We have disclosed in our earlier patent application two methods of modification of TS-1 molecular sieves, primarily for the vapor phase epoxidation of propylene and hydrogen peroxide. One of the methods is to treat the TS-1 molecular sieve with a mixed solution of tetrapropyl ammonium hydroxide (TPAOH) and inorganic salts (lithium, sodium, potassium and mixtures thereof) (Chinese patent application No. 201110338224.x), and the other method is to treat the TS-1 molecular sieve with a mixed solution of a halide of tetrapropyl quaternary ammonium cation and an inorganic base (hydroxide of alkali metals lithium, sodium and potassium) (Chinese patent application No. 201110338451.2, U.S. Pat. No. US9,486,790B2). In subsequent studies, we found that the above-mentioned invention patents applied in our earlier period have both applicability to liquid phase epoxidation of propylene and hydrogen peroxide, so that there are limitations in improving the epoxidation performance of propylene and hydrogen peroxide (the highest propylene conversion rate of the modified TS-1 molecular sieve in gas phase epoxidation reaction is less than or equal to 9%). Alternatively, neither of the two disclosed modification methods is capable of generating the TS-1 molecular sieve with efficient active centers most suitable for the phase epoxidation reaction of propylene and hydrogen peroxide. The common feature of the above patents is that it is emphasized that the residual alkali metal ions in the modified TS-1 molecular sieve are not favorable for achieving the modification effect. Therefore, in order to avoid the adverse effect of the remaining alkali metal ions as much as possible, it is clearly specified in the post-treatment step of the technical proposal that the TS-1 molecular sieve is hydrothermally treated with a mixed solution containing alkali metal ions, then sufficiently washed with deionized water, and the pH of the filtrate is made to be less than 9 (in the examples, it is indicated that pH 7 is the most preferable).

Disclosure of Invention

In order to further improve the technical level of the propylene and hydrogen peroxide gas phase epoxidation reaction, the invention provides an alkali metal ion modified titanium silicalite TS-1 capable of selectively promoting the propylene and hydrogen peroxide to perform epoxidation reaction in a gas phase state without participation of a solvent and a preparation method thereof.

The core of the invention is that the catalyst contains a small amount of tetrapropyl quaternary ammonium cation (TPA)⁺) The alkali metal hydroxide solution of (A) is used for carrying out hydrothermal treatment on the titanium silicalite TS-1 to a controlled extent, alkali metal cations have to be retained on the titanium silicalite after the hydrothermal treatment, and at least a part of the alkali metal cationsThe microenvironment of the framework titanium is modified on the silicon hydroxyl groups near the framework titanium in a balanced cation form. Small amounts of tetrapropyl quaternary ammonium cations (TPA) contained in alkali metal hydroxide solutions⁺) The method is used for recrystallizing a large amount of titanium silicalite molecular sieve dissolved substances generated in the hydrothermal modification process of the alkali metal hydroxide into the titanium silicalite molecular sieve. Modifying residual TPA in molecular sieves⁺The ions may be removed by calcination. The microenvironment at least comprises the electron cloud distribution and the geometric space factors of framework titanium. The alkali metal hydroxide is preferably sodium hydroxide and potassium hydroxide, and lithium hydroxide may be selected. Said TPA⁺The ions may conveniently be introduced via chloride (TPACl) and bromide (TPABr). We have surprisingly found, through repeated studies, that this alkali metal ion modification, although having little effect on the liquid phase epoxidation of propylene using methanol as a solvent at low temperatures, has an unexpectedly improved effect on the liquid phase epoxidation of propylene and hydrogen peroxide at high temperatures (typically > 100 ℃ at atmospheric pressure) in the absence of a solvent. For the gas-phase epoxidation reaction of propylene and hydrogen peroxide, the skeleton titanium active center of which the microenvironment is modified by alkali metal ions can enable the catalyst to obviously inhibit the self-decomposition side reaction of the hydrogen peroxide under the normal propylene/hydrogen peroxide feeding proportion, improve the propylene conversion rate, further improve the effective utilization rate of the hydrogen peroxide, reduce the oxygen generation and greatly improve the economy and safety of the gas-phase epoxidation reaction.

The embodiments and examples of the present invention are primarily developed around TS-1 molecular sieves. This is mainly because TS-1 is most representative of the family of titanium silicalite molecular sieves. It is relatively easy to synthesize, most common in the literature, and most industrially applicable. Currently, the TS-1 molecular sieve is actually used in the HPPO process of liquid-phase epoxidation of propylene and hydrogen peroxide. It is worth mentioning that in the present invention, a small amount of TPA⁺The use of ions not only reduces the modification loss of the titanium-silicon molecular sieve, but also greatly reduces the content of silicate dissolved substances in the alkali metal hydroxide waste liquid generated after modification, so that the discharged waste water is easier to treat and reaches the discharge standard, thereby the implementation of the modification method of the invention is changedIt is relatively easy. More significantly, a small amount of TPA⁺The use of the ions also solves the problem of applicability of the nanometer TS-1 molecular sieve matrix, and expands the application range of the alkali metal hydroxide solution hydrothermal modification. The invention is not only suitable for the micron TS-1 titanium silicalite molecular sieve synthesized by the non-classical method or the small-grain TS-1 molecular sieve parent body, but also suitable for the nanometer TS-1 molecular sieve (the size of the aggregate is generally below 200-300 nanometers) synthesized by the classical method introduced by Taramasso et al (USP4410501.1983) or Thangaraj et al (J Chem Soc Chem Commun, 1992: 123).

In the alkali metal ion modified TS-1 molecular sieve prepared by the invention, the skeleton titanium active center modified by the alkali metal ions has unique infrared spectrum characteristics, and the characteristic absorption peak of the vibration spectrum appears above 960cm^-1And less than 980cm^-1In a range different from the known four-coordinate skeletal titanium active center (infrared absorption is at 960 cm)^-1) And a six-coordinate skeleton titanium active center (Ti (OSi))₂(OH)₂(H₂O)₂) (UV Raman absorption peak at 695cm^-1) The novel framework titanium active center. Although publications such as J.Catal.,1995,151,77-86 have reported that sodium exchange of TS-1 molecular sieves with 1M NaOH solution at 25 ℃ can also change the infrared spectral characteristics of TS-1 molecular sieve framework titanium (960 cm^-1The absorption peak became 985cm^-1Shouldering) and it has been recognized that the sodium exchange that occurs in such strongly alkaline solutions is what is known as an exchange reaction of sodium ions in sodium hydroxide with hydrogen protons on silicon hydroxyl groups in the vicinity of framework titanium (NaOH + Si-OH → Si-O)^-Na⁺+H₂O). As a result, sodium ions exist as counter cations on the silicon hydroxyl groups in the vicinity of the framework titanium, which naturally changes the microenvironment of the framework titanium. However, in the present invention, the infrared characteristic absorption of the active center of the alkali metal ion-modified framework titanium occurs above 960cm^-1And less than 980cm^-1In a range of (1), which is in accordance with a value reported in the literature (985 cm)^-1) Differing by at least 5 wave numbers (cm)^-1) The above two types of infrared having such a large difference in wave number are not difficult for those familiar with infrared spectroscopy to understandAbsorbing vibrations that should belong to the titanium centers of different frameworks. In addition, it will be seen from the comparative examples provided herein that sodium exchanged TS-1 molecular sieves (framework titanium IR characteristic absorption at 985 cm) were made according to the methods reported in publications J.Catal.,1995,151,77-86^-1In the vicinity, consistent with literature reports) are essentially inactive towards the gas-phase epoxidation of propene and hydrogen peroxide. In sharp contrast, the sodium ion modified TS-1 molecular sieve obtained according to the invention (the infrared characteristic absorption of framework titanium appears above 960 cm)^-1And less than 980cm^-1In the range of (b) has high activity and high selectivity for the gas-phase epoxidation reaction. It is clear to those familiar with titanium silicalite molecular sieves that the first adjacent framework titanium has four framework silicons that are not spatially equivalent, and that the total number of possible hydroxyl sites on these four framework silicons is as many as 16, which differ more greatly from the center of the framework titanium. We confirm by means of quantum chemical calculations that the framework titanium infrared spectrum characteristics of the alkali metal ion modified TS-1 molecular sieve provided by the invention are different from those of the room-temperature sodium exchange TS-1 molecular sieve reported in the literature because two different methods cause the alkali metal ions to be combined at different positions of silicon hydroxyl groups.

The said invention uses TPA with small content⁺The ionic alkali metal hydroxide solution carries out hydrothermal treatment on the titanium silicalite molecular sieve in a controlled degree, wherein the term of controlled degree is mainly used for explaining that the hydrothermal treatment method provided by the invention can ensure that alkali metal ions always fall on the most favorable silicon hydroxyl positions, so that the skeleton titanium center microenvironment of the modified TS-1 molecular sieve is modified most favorably, and the gas-phase epoxidation reaction of propylene and hydrogen peroxide is promoted more effectively.

Thus, in summary, the present invention is characterized by containing a small amount of TPA⁺The hydrothermal treatment of the TS-1 molecular sieve with the ionic alkali metal hydroxide solution is controlled in degree, and after the hydrothermal treatment, the alkali metal cations must remain on the TS-1 molecular sieve, with at least a portion of the alkali metal cations in equilibrium cationic form on the appropriate silicon hydroxyls in the vicinity of the framework titanium for moderate modification of the microenvironment of the framework titanium. And alkali goldThe active center of the framework titanium modified by the metal ions can generate more than 960cm^-1And less than 980cm^-1Characteristic infrared absorption of (1).

To achieve the degree of controlled hydrothermal treatment according to the invention, it is first of all to carry out the hydrothermal treatment with a low-concentration alkali metal hydroxide solution. Secondly, when the TS-1 molecular sieve is hydrothermally treated by using a low-concentration alkali metal hydroxide solution, proper hydrothermal treatment temperature, time and liquid-solid ratio must be adopted. That is, the concentration of the alkali metal hydroxide solution, the hydrothermal treatment temperature, the time, and the liquid-solid ratio are basic parameters for controlling the degree of the hydrothermal treatment.

As mentioned above, the present invention is not only suitable for the non-classically synthesized micro TS-1 titanium silicalite molecular sieve or small crystal TS-1 molecular sieve precursor, but also suitable for the classically synthesized nano TS-1 molecular sieve (aggregate size is generally below 200-300 nm) introduced by Taramasso et al (USP4410501.1983) or Thangaraj et al (J Chem Soc Chem Commun, 1992: 123). The scientific principle obtained by research is as follows: the hydrothermal modification of the alkali metal hydroxide solution itself is fundamentally a dissolution modification. Therefore, the method is only modified by the alkali metal hydroxide solution, and cannot be applied to the nano TS-1 molecular sieve synthesized by the classical method and with small crystal grain size. This is because the TS-1 molecular sieve with too small a crystallite size is likely to be excessively dissolved or even completely dissolved in the modification, so that the required alkali metal modified framework titanium active center cannot be generated. In the present invention, a small amount of TPA contained in the alkali metal hydroxide solution⁺The ions can be enriched on the outer surface of the molecular sieve crystal in the hydrothermal modification process and play the role of a template agent. As a result, silicate species dissolved by the alkali metal hydroxide in the molecular sieve are transferred to the outer surface of the molecular sieve for recrystallization, so that a dynamic equilibrium process of dissolution inside crystal grains and recrystallization outside the crystal grains is established, the problem that the micrometer TS-1 molecular sieve is excessively lost due to hydrothermal modification by the alkali metal hydroxide solution is avoided, and the problem that the nanometer TS-1 molecular sieve cannot be suitable for the method because the nanometer TS-1 molecular sieve is excessively dissolved or even completely dissolved in the modification process is solved.

As mentioned above, the raw material characteristic of the TS-1 molecular sieve synthesis technology by the classical method is that tetrapropylammonium hydroxide is taken as a template agent, and silicon ester and titanium ester are taken as a silicon source and a titanium source respectively. The morphology of the product is observed on an electron microscope, the morphology is irregular aggregate morphology, the particle size of the aggregate is usually 200-300 nanometers, and the grain size of the primary crystal forming the aggregate is usually less than 100 nanometers. Although the later people do a lot of significant improvement work on the basis of Taramasso et al and Thangaraj et al, the basic characteristics of the TS-1 molecular sieve synthesized by the classical method are not changed, and the related documents are many. The following documents are incorporated by reference: USP 4410501; j Chem Soc Chem Commun, 1992: 123; J.Catal.,1995,151, 77-86; chinese invention patent (application No.) 02125738.8; chinese invention patent (application number) 201610160261.9.

As is well known to those skilled in the art, inexpensive TS-1 can be synthesized by various methods. The hydrothermal synthesis of inexpensive TS-1 is reported as in the following publications: zeolite and Related microporus Maierials State of the Art 1994, students in Surface Science and Catalysis, Vol.84; zeolite 16: 108-; zeolite 19: 246-; applied Catalysis A, General 185(1999) 11-18; catalysis Today 74(2002) 65-75; ind, hem, res, 2011,50, 8485-; microporous and Mesoporous Materials 162(2012) 105-114; chinese patent application No. 201110295555.x and 201110295596.9. The raw material of cheap synthesis technique is characterized by that it uses tetrapropyl ammonium bromide as template agent and uses ammonia water or organic amines of methylamine, ethylamine, ethylenediamine, diethylamine, n-butylamine and hexanediamine as alkali source. The silicon source and the titanium source are commonly used as silica sol and titanium tetrachloride, and sometimes titanium ester is used as the titanium source. The morphology of the product observed on an electron microscope is characterized by monodisperse crystals with regular crystal edge crystal faces, including large-grain thin plate crystals reaching several microns or coffin-like small-grain crystals of 300-600 nanometers.

Although the invention has no limitation on the grain size of the titanium-silicon molecular sieve parent, the titanium-silicon molecular sieve is required to have relatively low silicon-titanium ratio and non-framework titanium content as little as possible. These two points are easily understood. First, vapor phase epoxidation of propene and hydrogen peroxide requires a titanium silicalite molecular sieveThere is a higher density of titanium active sites which is beneficial in avoiding inefficient thermal decomposition of hydrogen peroxide. Secondly, as mentioned before, in the hydrothermal modification process provided by the present invention, the controlled dissolution of the alkali metal hydroxide solution takes place inside the crystals, while a small amount of TPA is added⁺The templating agent-directed recrystallization by the ions occurs at the outer surface of the grains. Therefore, if the titanium silicalite molecular sieve matrix contains too much non-framework titanium, especially when the non-framework titanium is distributed in a large amount in the interior of the crystal of the molecular sieve, although the total ratio of silicon to titanium seems suitable, it is not favorable for the modified titanium silicalite molecular sieve to contain enough framework titanium active centers modified by alkali metal ions.

In addition, the present invention also requires that the titanium silicalite precursor have as high a relative crystallinity as possible. This is not difficult to understand. After all, the crystal framework is the support of framework titanium.

The technical scheme of the invention is as follows:

the alkali metal ion modified titanium silicalite TS-1 is used for the hydrogen peroxide gas phase epoxidation reaction of propylene, in the alkali metal ion modified titanium silicalite TS-1, alkali metal ions are retained on silicon hydroxyl groups of the modified TS-1 molecular sieve, and the infrared characteristic absorption band of the skeleton titanium active center modified by the alkali metal ions is higher than 960cm^-1And less than 980cm^-1Within the range of (1); the TS-1 molecular sieve parent body of the alkali metal ion modified titanium-silicon molecular sieve TS-1 meets the following requirements: the molar ratio of silicon to titanium is less than or equal to 200; the index value of the framework titanium content is more than or equal to 0.40; the relative crystallinity is more than or equal to 85 percent.

Furthermore, the mole ratio of silicon to titanium of the parent body of the TS-1 molecular sieve is less than or equal to 100; the index value of the framework titanium content is more than or equal to 0.45; the relative crystallinity is more than or equal to 90 percent.

Wherein, the silicon-titanium molar ratio refers to the overall average silicon-titanium ratio of the sample in bulk. The silicon to titanium molar ratio data can be obtained by standard analysis using X-ray fluorescence spectroscopy (XRF). One skilled in the art can freely determine or commission the silicon to titanium ratio data for the TS-1 precursor according to XRF instrument specifications.

Wherein the index value of the framework titanium content is defined as I_960cm-1/I_550cm-1I.e. 960cm on TS-1 molecular sieve framework vibration infrared spectrum^-1The peak intensity of absorption of Ti-O-Si antisymmetric stretching vibration on the skeleton is represented, and the peak intensity is 550cm^-1The ratio of the absorption peak intensities of the five-membered ring vibration of the MFI structure is represented. As is well known to those skilled in the art, this ratio is generally accepted by researchers in the art to reflect the relative amount of framework titanium in TS-1 molecular sieves (e.g., the publication CATAL. REV. -SCI. ENG.,39(3).209-251(1997) using I)_960cm-1/I_550cm-1The values give the correlation map p217fig.4b). I is_960cm-1/I_550cm-1The larger the value, the more the framework titanium is contained in the TS-1 framework. The person skilled in the art can refer to the experimental method of the framework vibration infrared spectrum of the titanium silicalite molecular sieve described in any published documents to obtain I_960cm-1/I_550cm-1The value is obtained. The present invention provides the following for reference: predrying spectrally pure KBr at 110 ℃ for 4 hours, mixing and grinding the KBr and the TS-1 molecular sieve into powder according to the proportion of 100-200: 1, pressing the powder into a sheet under the pressure of 6MPa, and placing the sheet into an infrared sample bin for testing. 960cm^-1And 550cm^-1The peak intensities of the two absorption peaks can be directly read from the spectrogram by self-band software of a spectrometer, so that I can be conveniently calculated_960cm-1/I_550cm-1) The value is obtained.

Wherein, the relative crystallinity refers to the ratio (expressed as percentage) of the sum of five characteristic diffraction peaks (2 theta ═ 7.8 °, 8.8 °, 23.0 °, 23.9 ° and 24.3 °) of the TS-1 molecular sieve precursor and the reference sample, as measured by X-ray polycrystalline powder diffraction method (XRD). The XRD diffraction patterns of the parent TS-1 molecular sieve and the reference sample can be obtained by a person skilled in the art according to the XRD experimental method reported in any published documents. The present invention recommends the preparation of a reference sample using example 1 of chinese invention patent (application No.) 201110295555. x. The method comprises the following specific steps: adding 220 ml of deionized water into 225 g of silica sol (20% wt), stirring for 10 minutes, adding 18.4 g of tetrapropyl ammonium bromide and 5.1 g of treated seed crystal into the silica sol, and continuously stirring for 20 minutes to obtain a raw material silicon solution; mixing tetrabutyl titanate and acetylacetone in a mass ratio of 1: 0.8, and stirring for 15 minutes to prepare a raw material titanium solution; taking 19.7 ml of the prepared raw material titanium solutionAdding the solution into a raw material silicon solution, stirring for 30 minutes, adding 57 ml of n-butylamine, and continuously stirring for 15 minutes to obtain uniform gel; 6.0 g of Na was added₂SO₄Stirring for 10 minutes; then adding the obtained gel into a 2L stainless steel reaction kettle, and crystallizing for 24 hours under the autogenous pressure and the temperature of 170 ℃; the product was filtered, washed to neutrality and dried at 110 ℃. It is emphasized that before the XRD diffractograms of the parent TS-1 molecular sieve and the reference sample are determined, the two samples to be tested must be baked to ensure that the organic template in the samples is removed completely and the dry basis content of the molecular sieve is more than 95%, preferably more than 98%. For this purpose, it is recommended to take about 2g of each of the TS-1 molecular sieve precursor and the reference sample, dry the precursor overnight at 110 ℃ and then place the sample in a muffle furnace for temperature-programmed calcination. The temperature programmed calcination is started from room temperature, and is carried out at a heating rate of 10 ℃/min to 300 ℃, then, the temperature is increased from 300 ℃ to 500 ℃ at a heating rate of 1 ℃/min, and the temperature is kept until the sample is completely whitened.

The TS-1 molecular sieve which meets the indexes can be used as the modified parent of the invention. The TS-1 molecular sieves suitable for use in the present invention are commercially available or can be synthesized by engineers familiar with the art from relevant publications and patent literature. If self-synthesized, the invention recommends adopting the TS-1 molecular sieve hydrothermal synthesis method reported in the following publications and patent documents:

wherein, the micron TS-1 molecular sieve can be synthesized by adopting the method reported in the following documents: zeolite and Related microporus Maierials State of the Art 1994, students in Surface Science and Catalysis, Vol.84; zeolite 16: 108-; zeolite 19: 246-; applied Catalysis A, General 185(1999) 11-18; catalysis Today 74(2002) 65-75; ind, hem, res, 2011,50, 8485-; microporous and Mesoporous Materials 162(2012) 105-114; chinese patent application No. 201110295555.x and 201110295596.9.

Wherein, the nanometer TS-1 molecular sieve can be synthesized by adopting the method reported in the following documents: USP 4410501; j Chem Soc Chem Commun, 1992: 123; J.Catal.,1995,151, 77-86; chinese invention patent (application No.) 02125738.8; chinese invention patent (application number) 201610160261.9.

Of course, the engineers familiar with the art can well synthesize the TS-1 molecular sieve precursors for use in the present invention according to other suitable known methods.

The organic template agent of the qualified TS-1 molecular sieve parent body needs to be removed before modification. The removal of organic templating agents from molecular sieves is common knowledge in the art. The invention provides the following reference method: taking a proper amount of TS-1 molecular sieve matrix, drying the matrix at 110 ℃ overnight, and then placing the sample in a muffle furnace for temperature programming roasting. The temperature programmed roasting is started from room temperature, the temperature is raised to 300 ℃ at the temperature raising rate of 5 ℃/min, then the temperature is raised from 300 ℃ to 400 ℃ at the temperature raising rate of 1 ℃/min and is kept constant for 12 hours, then the temperature is raised to 450 ℃ at the same temperature raising rate and is kept constant for 12 hours, and finally the temperature is raised to 500 ℃ at the same temperature raising rate and is kept constant until the sample is completely whitened.

The preparation method of the alkali metal ion modified titanium silicalite TS-1 for the hydrogen peroxide gas phase epoxidation reaction of propylene comprises the following steps:

in the first step, a formulation containing a small amount of TPA⁺Ionic alkali metal hydroxide modification solutions. In order to achieve the effect of controlled hydrothermal modification, the invention requires that:

among them, the concentration of the alkali metal hydroxide solution is preferably in the range of 0.01 at the lower limit and 0.20 at the upper limit (room temperature), and more preferably in the range of 0.05 at the lower limit and 0.15 at the upper limit (room temperature).

Alkali metal hydroxides, preferably lithium hydroxide, sodium hydroxide and potassium hydroxide; more preferably sodium hydroxide and potassium hydroxide;

in preparing the modifying solution, any one of the alkali metal hydroxides recommended above may be used alone, a mixture of any two of the hydroxides may be used in any ratio, and a mixture of any three of the hydroxides may be used in any ratio. When two or more alkali metal hydroxides are used to prepare the modifying solution, the solution concentration is the sum of the molar concentrations of the respective hydroxides.

Wherein, TPA⁺The preferred range of the ion concentration is a lower limit of 0.05 and an upper limit of 0.50 mol/liter (room temperature), more preferably in a range of 0.10 at the lower limit and 0.3 mol/liter (room temperature) at the upper limit.

TPA⁺The ions are preferably provided by tetrapropylammonium chloride (TPACl), tetrapropylammonium bromide (TPABr). When preparing the modifying solution, any one of the quaternary ammonium salts recommended above may be used alone, or a mixture of two quaternary ammonium salts may be used in any ratio.

It is stated herein that TPA is provided using other tetrapropyl quaternary ammonium salt precursor compounds such as tetrapropyl ammonium fluoride (TPAF) and tetrapropyl ammonium iodide (TPAI)⁺But also possible. However, with TPAF and TPAI, it is possible to bring about treatment F^-And I^-The trouble of the ions and therefore the use of the present invention is not recommended. In addition, although Tetraethylammonium (TEA)⁺) And Tetrabutylammonium (TBA)⁺) It also has templating effect on oriented synthesis of MFI molecular sieve topology, but they are inferior to TPA in either price or ability to oriented crystallization of all-silicon MFI molecular sieve framework⁺Ions, and therefore, are not recommended.

Since commercially available alkali metal hydroxides contain a certain amount of impurities, chemical titration analysis of the purity of the alkali metal hydroxide raw material is carried out before preparing the modified solution. The titration procedure can be performed by one skilled in the art according to conventional chemical analysis methods. Similarly, after the modifying solution is prepared, the hydroxide concentration of the modifying solution is calibrated by the same conventional chemical analysis method. Since sodium hydroxide gives off a large amount of heat during the dissolution process, concentration calibration can only be performed after the solution is cooled to room temperature.

The fresh modifying solution can react with the molecular sieve skeleton (e.g., NaOH + Si-OH → Si-O) during modification^-Na⁺+H₂O) and also contain low concentrations of silicates, titanates and titanosilicates due to framework dissolution desilication reactions (during which small amounts of framework titanium are also inevitably dissolved). Simultaneously, TPA⁺The ions also act as a templating agent in the recrystallization of the molecular sieve crystal surface, and a portion of the TPA⁺The ions are encapsulated in the newly grown molecular sieve framework, so that the concentration of the ions in the modifying solution is reducedAnd (4) degree. However, it is needless to say that the used alkali metal hydroxide-modified residual liquid can be conditionally recycled, which can reduce the modification cost and the waste liquid discharge. Before recycling the modified raffinate, the alkali metal hydroxide and TPA need to be accurately measured⁺The concentration of the ions is adjusted to restore the original concentration by adding, and the concentrations of the silicate, titanate and silicotitanate contained in the solution are measured to control the number of recycling times. In order to avoid over-complicating and verbose the description of the invention and to facilitate the understanding of the gist of the invention, the invention is not described in any greater way in this and the following examples with respect to the recycling of the modifying solution, which can be carried out by the engineers in the field on the basis of common general knowledge.

It is stated, however, that just as the used modifying solution can be recycled, it is also possible to artificially add certain amounts of alkali metal silicates, titanates and titanosilicates in order to help control the extent of dissolution modification when formulating fresh modifying solutions. Even we have found that the proper introduction of alkali metal carbonates and bicarbonates into fresh modifying solutions also has the effect of assisting in controlling the degree of dissolution modification when preparing the same. The common property of the salts is that the salts belong to strong alkali and weak acid salts, and the alkali metal cation and hydroxide anion can be provided by hydrolysis in aqueous solution, namely, the alkali metal hydroxide is actually generated by hydrolysis. The difference between the method and the method for preparing the modified liquid by directly adding the alkali metal hydroxide is that weak acid is generated after the weak acid salt is hydrolyzed, and the pH value (pH value) of the modified liquid can be adjusted to a certain degree, so that the method is favorable for controlling the degree of dissolution modification. The alkali metal phosphate and the hydrogen phosphate can also be properly introduced into the modification liquid from the hydrolysis characteristic of the strong alkali and the weak acid salt, but the phosphate and the hydrogen phosphate are easy to accumulate in the cyclic use of the modification liquid, and are not beneficial to the cyclic use of the modification liquid for multiple times. As for the addition of alkali metal salts of other strong bases and weak acids, the skilled engineers can choose them according to the above principles of the invention, and the details are not repeated.

In the second step, a catalyst containing a small amount of TPA is used⁺The TS-1 molecular sieve is subjected to controlled hydrothermal treatment by an ionic alkali metal hydroxide solution. The hydrothermal treatment may be at restAnd under stirring. In order to achieve the effect of controlled hydrothermal treatment, the invention requires that:

the preferable proportion range of the dosage (volume) of the modification liquid and the dosage (mass) of the parent material of the titanium-silicon molecular sieve is 5 ml/g molecular sieve at the lower limit and 15 ml/g molecular sieve at the upper limit, and the more preferable proportion range is 8 ml/g molecular sieve at the lower limit and 12 ml/g molecular sieve at the upper limit;

the preferable range of the hydrothermal modification temperature is lower limit 100 ℃ and upper limit 200 ℃, and the more preferable range is lower limit 150 ℃ and upper limit 190 ℃;

the hydrothermal modification time is preferably within a range of a lower limit of 10 hours and an upper limit of 20 hours, and more preferably within a range of a lower limit of 15 hours and an upper limit of 20 hours.

It should be noted that, in order to achieve the effect of controlled hydrothermal treatment, all parameters of the concentration of the alkali metal hydroxide modification solution, the proportion of the amount of the modification solution to the amount of the titanium silicalite molecular sieve parent material, the modification temperature and the modification time need to be considered at the same time. It will be understood by those skilled in the art that the lower limit of each hydrothermal modification parameter involved in the second and third steps is the weakest modification effect and the upper limit of each hydrothermal modification parameter is the strongest modification effect. Therefore, the modification conditions using the combination of the lower limit values of all the parameters necessarily yield the lowest modification results, and the modification conditions using the combination of the upper limit values of all the parameters necessarily yield the highest modification results. It will also be understood that the combination of the lower limit of a certain parameter with the intermediate and upper limits of the remaining parameters will yield a minimum and maximum hydrothermal modification. Of course, it is also understood that the modification conditions resulting from the combination of different values of the parameters may produce varying degrees of hydrothermal modification. When the values of all parameters are combined with the modification degree required to be achieved by the specified titanium silicalite molecular sieve parent body, the controlled hydrothermal modification is pointed by the invention. Obviously, the minimum and maximum modification effects referred to herein cannot be erroneously understood as the worst and best modification effects. Some titanium silicalite precursors require minimal hydrothermal modification, while some require higher hydrothermal modification. Therefore, the invention clarifies that what combination conditions are adopted for carrying out hydrothermal modification aiming at a specific titanium silicalite molecular sieve parent body is determined by experiments, and the judgment is based on the infrared vibration absorption peak position of an active center, particularly the gas-phase epoxidation reaction data of propylene and hydrogen peroxide.

Engineers in the field can determine specific modification conditions suitable for a given titanium silicalite molecular sieve within the parameter value range recommended by the invention according to specific considerations such as equipment use efficiency, modification cost, wastewater discharge and the like.

And thirdly, carrying out post-treatment on the hydrothermally modified TS-1 molecular sieve. The method specifically comprises the steps of conventional solid-liquid separation, washing, drying and roasting. For the invention, the key to the correct washing of the wet molecular sieve material after solid-liquid separation is the most important. The invention recommends that the modified molecular sieve wet material obtained by solid-liquid separation is washed by using alkali metal hydroxide solution with low concentration, and the washing degree is based on that no precipitate appears after the washing liquid is neutralized by acid and alkali. It is claimed in the invention that the concentration of alkali metal hydroxide solution used for washing purposes preferably ranges from a lower limit of 0.001 and an upper limit of 0.05 mol/l (room temperature calibration), more preferably ranges from a lower limit of 0.005 and an upper limit of 0.04mol/l (room temperature calibration). More preferably, the lower limit of the range is 0.005 and the upper limit is 0.03mol/l (room temperature calibration). Said alkali metal hydroxide is preferably lithium hydroxide, sodium hydroxide and potassium hydroxide; more preferred are sodium hydroxide and potassium hydroxide.

The necessity of washing is, on the one hand, that the molecular sieve wet material obtained from the solid-liquid separation still retains a large amount of the modified raffinate. It is in the form of surface liquid film and capillary pore condensate, and can be used for making wet material, and its weight ratio is 40-50%. The modified residual liquid contains more free alkali metal hydroxide, silicate ions, titanate ions, silicotitanate ions and larger molecular sieve skeleton fragments which are dissolved from the molecular sieve skeleton, wherein excessive free alkali can continuously react with silicon hydroxyl groups in the drying process to destroy the expected modification degree, and other species can become the blockage of molecular sieve pores and even become active centers for initiating various side reactions. On the other hand, washingImproper washing methods and levels tend to result in the loss of the useful alkali metal ions that are balanced on the silicon hydroxyl groups. Therefore, it is very important to select the correct washing method and degree. The alkali metal hydroxide solution used for washing purposes in the present invention is an alkali metal hydroxide solution having a concentration much lower than that of the modifying solution, and the simplest method is to use an alkali metal hydroxide solution of the same type as the modifying solution but having a lower concentration as the washing solution. With regard to the selection of alkali metal hydroxide as the washing solution, the present invention provides the following reasons: first, the use of an alkali metal hydroxide solution as the washing solution facilitates the replenishment of the available alkali metal ions that are balanced on the silicon hydroxyl groups that are lost during the washing process. Secondly, the alkali metal hydroxide solution is strongly alkaline, and the strong alkalinity of the washing liquid can be maintained in the washing process to prevent the loss of useful alkali metal ions balanced on the silicon hydroxyl groups. Otherwise, if deionized water is selected as the wash solution, then due to the presence of the retro NaOH + Si-OH → Si-O^-Na⁺+H₂O reaction, i.e. strong alkali weak acid salts (Si-O)^-Na⁺) So that the useful alkali metal ions in equilibrium on the silicon hydroxyl groups are easily lost in the form of NaOH. This is the reason why the conventional patents relating to the modification with inorganic bases can remove alkali metal ions in the water washing step. Therefore, in order to obtain the TS-1 molecular sieve modified by the alkali metal ions, deionized water is not suitable, and a solution with an acidic pH value cannot be used as a washing liquid of wet materials after solid-liquid separation. It goes without saying that.

Of course, it is also feasible in principle to use low concentrations of quaternary ammonium bases or other more strongly basic organic bases as washing solutions, but this is not desirable from an environmental and cost standpoint.

In addition, the invention emphasizes that the alkali metal hydroxide solution with the concentration lower than that of the modification solution is used as the washing solution, and aims to reduce the residual amount of free alkali metal hydroxide in the modified TS-1 molecular sieve after washing. The detriment of excessive free alkali metal hydroxide remaining in the modified TS-1 molecular sieve has been previously explained.

The most simple and applicable modes of solid-liquid separation in the invention are centrifugation and filtration. However, when the centrifugation and filtration mode is used, additives such as flocculating agent, filter aid and the like are not introduced as much as possible so as to prevent the pH value of the solution from changing and even causing precipitates to appear. Other solid-liquid separation methods are also acceptable as long as they do not cause significant concentration of the liquid phase, precipitate, or change in pH during the separation.

The drying and baking of the invention can be carried out in the air atmosphere according to the conventional method. The reference method recommended by the invention is as follows: the drying temperature range is 80-120 ℃, and the drying time is selected according to the dry basis content of the sample not less than 90%. The recommended roasting final temperature range is 400-550 ℃, and the constant temperature time at the final temperature is not less than 3 hours

The effects of the present invention can be evaluated by the following means:

firstly, the position of an absorption peak of an active center of the modified TS-1 molecular sieve catalyst is represented by an infrared spectrum. The method comprises the following steps: and (3) taking a proper amount of sample from the modified TS-1 molecular sieve subjected to the post-treatment in the third step, putting the sample into one small beaker, simultaneously taking a proper amount of spectrally pure KBr, putting the spectrally pure KBr into the other small beaker, and pre-drying the two small beakers in a 110-DEG oven for 4 hours. Then mixing KBr and TS-1 molecular sieve according to the proportion of 200:1, grinding into powder, and pressing into slices under the pressure of 6 MPa. And then placing the slice into an infrared sample bin for testing to obtain an infrared spectrogram. And finally, accurately positioning the infrared characteristic absorption peak position of the alkali metal ion modified framework titanium active center by using a second derivative spectrum in infrared software.

And secondly, analyzing the modified sample by using an X-ray fluorescence spectrum (XRF) method to obtain silicon-titanium molar ratio and sodium ion content data.

In addition, the small fixed bed reactor was used to evaluate the gas phase epoxidation performance of the modified TS-1 molecular sieve catalyst. It is suggested to evaluate the epoxidation of propylene and hydrogen peroxide in the gaseous phase by reference to the experimental set-up and methods described in our journal articles published previously and in the issued patents of the chinese invention. The references proposed by the present invention include: chem. Commun.,2005, 1631-; modern chemical industry, vol.26 supplement, 2006, P194-197; AIChE J,53: 3204-; new electrical and energy technology, Vol28,2009, No.3, P73-76; chi.j.cat., 2010,31: 1195-; the chemical bulletin Vol63,2012, No.11, P3513-3518; journal of Catalysis 288(2012) 1-7; angew.chem.int.ed.2013,52, 8446-; AIChE J,64: 981-; chinese invention patent (application number) 200310105210.9,200310105211.3,200310105212.8.

The evaluation method is characterized in that an integrated reactor is adopted. The upper section of the integrated reactor is a self-cooling medium barrier discharge reactor which is used for synthesizing gaseous hydrogen peroxide from hydrogen and oxygen plasmas in situ; the lower section of the integrated reactor is a conventional fixed bed reactor, titanium silicalite molecular sieve particles (20-40 meshes) are filled in the reactor, and the reactor is used for gas-phase epoxidation reaction of propylene and hydrogen peroxide gas. The working principle of the integrated reactor is as follows: the hydrogen and oxygen were mixed at gas rates of 170 ml/min and 8 ml/min respectively under the control of mass flow controllers and then fed into a self-cooled dielectric barrier discharge reactor in the upper section of the integrated reactor to synthesize gaseous hydrogen peroxide, with a hydrogen peroxide yield of 0.35 g/hr. The synthesized hydrogen peroxide gas enters a lower-section epoxidation reactor from an air hole between two sections of reactors under the carrying of excess hydrogen, is fully mixed with propylene gas (18 ml/min) entering the section of reactor from the side line, and enters a TS-1 catalyst bed layer together for epoxidation reaction.

The reaction conditions are as follows: the dosage of the TS-1 catalyst is 0.5 g (catalyst powder is crushed and sieved after being pressed into tablets to obtain 20-40 meshes), the actual molar ratio of the propylene to the hydrogen peroxide is about 5:1, and the gas-phase epoxidation reaction is carried out at normal pressure and 130 ℃.

The invention has the beneficial effects that: the invention uses a composition containing a small amount of tetrapropyl quaternary ammonium cation (TPA)⁺) The alkali metal hydroxide solution carries out hydrothermal treatment on the titanium silicalite TS-1 in a controlled degree, alkali metal cations must be retained on the titanium silicalite after the hydrothermal treatment, and at least a part of the alkali metal cations are in a balanced cation form and are positioned on silicon hydroxyl groups near the framework titanium to modify the microenvironment of the framework titanium. The alkali metal ion modification has an unexpected improvement effect on the propylene and hydrogen peroxide gas phase epoxidation reaction under the conditions of no solvent and high temperature (generally more than 100 ℃ under normal pressure). For propylene and peroxideIn the gas-phase epoxidation reaction of hydrogen, the skeleton titanium active center of which the microenvironment is modified by alkali metal ions can enable the catalyst to obviously inhibit the self-decomposition side reaction of hydrogen peroxide under the normal propylene/hydrogen peroxide feeding proportion, improve the propylene conversion rate, further improve the effective utilization rate of hydrogen peroxide, reduce the generation of oxygen and further greatly improve the economy and safety of the gas-phase epoxidation reaction. In addition, a small amount of TPA is introduced into the hydrothermal modification solution⁺The ions not only reduce the modification loss of the titanium-silicon molecular sieve, but also greatly reduce the content of silicate dissolved matters in the alkali metal hydroxide waste liquid generated after modification, so that the discharged waste water is easier to treat and discharge up to the standard, and the modification method of the invention is easier to implement. More significantly, a small amount of TPA⁺The use of the ions also solves the problem of applicability of the nanometer TS-1 molecular sieve matrix, and expands the application range of the alkali metal hydroxide solution hydrothermal modification.

Drawings

FIG. 1 is a graph of the FT-IR spectra of skeletal oscillations of samples of catalysts of example 1 and comparative example 2.

Figure 2 is an XRD diffractogram of a sample of the catalyst of example 4.

FIG. 3 is a graph of the FT-IR spectrum of skeletal vibrations of a sample of the catalyst of example 4.

FIG.4 is a FT-IR spectrum of skeletal vibration of a sample of the catalyst of example 5.

Detailed Description

The following examples are merely illustrative of the present invention. But do not limit the scope of the invention. All reagents and drugs mentioned in the examples are commercially available analytical grade.

And the SEM image is tested by adopting a NOVA NanoSEM 450 type field emission scanning electron microscope of FEI company in USA, the voltage is 230kV, the frequency is 60Hz, the current is 8A, the magnification is 800000-1600000 times, a sample is dispersed into absolute ethyl alcohol, a capillary is used for dropping on a silicon chip, then the sample is fixed on a conductive adhesive, and then gold spraying treatment is carried out on the conductive adhesive, and the analysis of the surface appearance X-ray polycrystalline powder diffraction (XRD) crystal structure is observed: measured by an X-ray powder diffractometer model D/max.2400 of Rigaku corporation, CuKa radiation, a voltage of 40kV, a current of 100mA, a scanning diffraction angle range of 2 theta of 4-40 DEG, a scanning speed of 2 DEG/min, and a scanning step of 0.08 deg. The relative crystallinity was determined from the ratio of the sum of the intensities of the five MFI structure characteristic peaks at 2 θ ═ 7.8 °, 8.8 °, 23.2 °, 23.8 ° and 24.3 ° in the XRD spectrum to the sum of the intensities of the five diffraction peaks of the standard (self-selected).

X-ray fluorescence spectroscopy (XRF) compositional analysis: 1.2g of a TS-1 sample and 4g of boric acid are uniformly mixed and prepared by adopting a German Bruker S8Tiger type X-ray fluorescence spectrometer, and the determination is carried out by adopting a non-standard method.

And (3) carrying out FT-IR spectrum TS-1 skeleton vibration characterization: the method IS carried out on an IS10 infrared spectrometer of Nicolet company, KBr tabletting IS adopted, the wave number range of scanning IS 4000-400 cm < -1 >, and the scanning times are 64 times.

EXAMPLE 1 this example illustrates that the present invention provides a composition containing a minor amount of TPA⁺The ionic large-grain micron TS-1 molecular sieve modified by the alkali metal hydroxide solution controlled hydrothermal treatment method has high activity and selectivity on the propylene and hydrogen peroxide gas phase epoxidation reaction and the effective utilization rate of hydrogen peroxide.

The first step is as follows: large-grain micron TS-1 molecular sieve precursors were prepared synthetically according to the method described in published application Appl. Catal.A,185, (1999) 11-18.

The specific material feeding amount and the synthesis steps are as follows:

adding 220 ml of deionized water into 225 g of silica sol (26% wt), stirring for 10 minutes, adding 18.4 g of tetrapropyl ammonium bromide into the glue solution, and continuously stirring for 20 minutes to obtain a raw material silicon solution; mixing tetrabutyl titanate and acetylacetone in a mass ratio of 1: 0.8, and stirring for 15 minutes to prepare a raw material titanium solution; adding 19.7 ml of the prepared raw material titanium solution into the raw material silicon solution, stirring for 30 minutes, adding 57 ml of n-butylamine, and continuously stirring for 15 minutes to obtain uniform gel; then the obtained gel is added into a 2L stainless steel reaction kettle, and the hydrothermal synthesis is carried out for crystallization for 96 hours under the stirring at the temperature of 170 ℃. After the crystallization time is up, the hydrothermal crystallization kettle is naturally cooled to room temperature, then the synthesis kettle is opened, and the mother liquor is separated by a Buchner funnel suction filtration method to obtain a molecular sieve filter cake. The filter cake was washed several times with deionized water until the pH of the water wash was approximately 7.0. The filter cake was then dried overnight in an electric oven at 110 ℃. And transferring the dried solid into a muffle furnace for temperature programmed roasting to remove the template agent. The temperature programmed roasting is started from room temperature, the temperature rising rate is10 ℃/min to 300 ℃, then the temperature rising rate is1 ℃/min, the temperature is increased from 300 ℃ to 500 ℃, and the temperature is kept until the sample is completely whitened, so that the large-grain micrometer TS-1(2) matrix is obtained.

To calculate the relative crystallinity of the large grain micron TS-1 precursor with the reference sample, the reference sample was prepared again with example 1 in chinese invention patent (application No.) 201110295555. x. The method comprises the following specific steps: adding 220 ml of deionized water into 225 g of silica sol (20% wt), stirring for 10 minutes, adding 18.4 g of tetrapropyl ammonium bromide and 5.1 g of treated seed crystal into the silica sol, and continuously stirring for 20 minutes to obtain a raw material silicon solution; mixing tetrabutyl titanate and acetylacetone in a mass ratio of 1: 0.8, and stirring for 15 minutes to prepare a raw material titanium solution; adding 19.7 ml of the prepared raw material titanium solution into the raw material silicon solution, stirring for 30 minutes, adding 57 ml of n-butylamine, and continuously stirring for 15 minutes to obtain uniform gel; 6.0 g of Na was added₂SO₄Stirring for 10 minutes; the resulting gel was then placed in a2 liter stainless steel reaction kettle and crystallized under autogenous pressure with stirring at 170 ℃ for 24 hours. The post-treatment method of the reference sample is carried out according to the processing method of the large-grain micrometer TS-1 molecular sieve matrix.

The crystal size of the large-crystal-grain micrometer TS-1 molecular sieve matrix is 1X 2X 5 mu m, the total Si/Ti molar ratio is about 34.5, and sodium ions are not detected by SEM, XRF, FT-IR and XRD respectively. The index value of the content of titanium in the skeleton, I960cm-1/I550cm-1, was about 0.50 and the relative crystallinity was about 100%. The measurement result shows that the synthesized large-grain micrometer TS-1 matrix meets the requirements of the invention.

The second step is that: the formulation contained 0.15mol/L TPA⁺0.1mol/L sodium hydroxide modified solution.

The solution was prepared with analytically pure sodium hydroxide solid particles (96%), tetrapropylammonium bromide (TPABr) and deionized water.

First, a 1 liter volumetric flask is taken and added with an accurate weight4.17 grams of sodium hydroxide and 40 grams of TPABr solid particles. Then formulated with a 1 liter volumetric flask containing 0.15mol/L TPA⁺And 0.1mol/L sodium hydroxide solution (cooled to room temperature). In the interest of caution, the prepared sodium hydroxide solution was calibrated by conventional procedures using reference reagents potassium hydrogen phthalate and phenolphthalein as indicators, and a relative deviation of concentration values of less than 5% was found to be a qualified solution. Otherwise, the modified solution is reconstituted.

The third step: with a composition containing 0.15mol/L TPA⁺The 0.1mol/L sodium hydroxide solution is used for carrying out controlled hydrothermal treatment on the TS-1 molecular sieve parent substance with micron and large crystal grains.

The specific method comprises the following steps: 50ml of calibrated TPA (terephthalic acid) containing 0.15mol/L is accurately measured by using a measuring cylinder⁺Adding 0.1mol/L sodium hydroxide solution into a plastic cup with magnetic stirring, and starting the magnetic stirring to slowly stir the solution. Then, 5 g of a large-grain micrometer TS-1 molecular sieve matrix which is whitened by roasting in the step one and completely removes the template agent is weighed and slowly poured into the stirred modified solution. After the large-grain micron TS-1 molecular sieve matrix is completely added into the solution, the stirring speed is properly increased to ensure that the slurry is in a uniform state. Stirring was continued for 2 hours at room temperature, then stirring was stopped and the slurry was transferred to a 100ml hydrothermal kettle and sealed. The hydrothermal kettle is put into an oven with the temperature of 170 ℃ and is kept constant for 16 hours.

The fourth step: and (3) carrying out post-treatment on the hydrothermally modified TS-1 molecular sieve.

After the hydrothermal treatment, the hydrothermal reactor was taken out of the electric oven and rapidly cooled to room temperature with tap water. Then the hydrothermal kettle is carefully opened, and the modified solution is filtered and separated by a Buchner funnel to obtain a molecular sieve filter cake. And (3) washing the filter cake by using 0.01mol/L sodium hydroxide solution until no precipitate appears after the filtrate is neutralized by acid and alkali, and stopping the washing operation. The filter cake was then dried overnight in an electric oven at 110 ℃ to ensure that the dry content of the solid powder (solids content measured by baking at 500 ℃ for 3 hours) was not less than 90%. Finally, the dried solid powder was calcined at 540 ℃ for 6 hours at constant temperature to yield approximately 4.46 g of the modified TS-1 molecular sieve product of example 1 with a product yield of 91 wt.%.

The sodium ion modified TS-1 molecular sieve prepared in this example was evaluated by the following tests:

firstly, the absorption peak position of the modified TS-1 molecular sieve framework titanium active center is represented by an infrared spectrum.

And taking a proper amount of the modified product obtained in the fourth step, putting the modified product into one small beaker, simultaneously taking a proper amount of spectrally pure KBr, putting the two small beakers into the other small beaker, and simultaneously putting the two small beakers into a 110-DEG oven for pre-drying for 4 hours. Then mixing and grinding the KBr and the modified TS-1 molecular sieve product according to the proportion of 200:1, and pressing the mixture into a sheet under the pressure of 6 MPa. And then placing the slice into an infrared sample bin for testing to obtain an infrared spectrogram. Finally, accurately positioning the infrared characteristic absorption peak position of the alkali metal ion modified framework titanium active center to be 978cm by using a second derivative spectrum in infrared software^-1。

In addition, the modified product obtained by analysis by X-ray fluorescence spectroscopy (XRF) as mentioned in the first step had a Si/Ti molar ratio of 33.6 and a Na/Ti molar ratio of 0.46.

The characterization test results of infrared spectrum and X-ray fluorescence spectrum show that TPA containing 0.15mol/L is used⁺The 0.1mol/L sodium hydroxide solution modifies the hydrothermal treatment of the TS-1 molecular sieve parent substance with micron and large crystal grains, and partial dissolution-recrystallization is generated, so that the silicon-titanium molar ratio of the modified catalyst is slightly reduced compared with the parent substance. Meanwhile, a large amount of sodium ions exist in the modified catalyst, and cause the infrared characteristic absorption peak of the active center of the framework titanium to be 962cm^-1(parent, FIG. 1A) to 978cm^-1Here (fig. 1B). That is, in the presence of 0.15mol/L TPA⁺In the process of carrying out controllable hydrothermal treatment modification on the micrometer large crystal grain TS-1 matrix by using 0.1mol/L sodium hydroxide solution, sodium ions replace hydrogen protons on silicon hydroxyl groups near framework titanium in a balanced cation form, and the microenvironment of the center of the near framework titanium is changed.

Then, the gas phase epoxidation performance of the modified TS-1 molecular sieve catalyst was evaluated using a small fixed bed reactor.

The gas phase epoxidation reaction experiments were carried out using an integrated reactor as reported in published article Chin.J.Catal.,2010,31: 1195-containing 119. The upper section of the reactor is a self-cooling medium barrier discharge reactor which is used for synthesizing gaseous hydrogen peroxide from hydrogen and oxygen plasmas in situ; the lower section of the integrated reactor is a conventional fixed bed reactor, titanium silicalite molecular sieve particles (20-40 meshes) are filled in the reactor, and the reactor is used for gas-phase epoxidation reaction of propylene and hydrogen peroxide gas. The specific operation steps are as follows: (1) calibration of the hydrogen peroxide yield of the upper stage plasma reactor: at this point the lower reactor section is removed. The self-cooling circulating water of the upper-stage reactor is firstly opened. Then, starting a hydrogen cylinder and a mass flow controller thereof, and controlling the hydrogen flow to be 170 ml/min; the oxygen cylinder was then opened and the oxygen flow was slowly increased to 8 ml/min using its mass flow controller. In the discharge reaction of the upper reactor, the flow rates of hydrogen and oxygen are accurately controlled and the hydrogen and oxygen are uniformly mixed before entering the upper reactor. Then, dielectric barrier discharge is carried out according to the discharge method introduced in the Chinese invention patent (application number) 200310105210.9,200310105211.3,200310105212.8, so that the hydrogen-oxygen mixed gas entering the upper segment self-cooling type dielectric barrier discharge reactor of the integrated reactor generates gaseous hydrogen peroxide through plasma reaction. The yield of the hydrogen peroxide is about 0.35 g/h by using a conventional iodometric method for calibration; (2) the two-stage reactor is used in an integrated manner for carrying out the gas-phase epoxidation of propene and hydrogen peroxide. After the calibration step, the discharge was stopped first, then the oxygen was stopped, and then the hydrogen was stopped after 10 minutes. 0.5 g of modified large-grain micrometer TS-1 molecular sieve catalyst (which is previously tableted, crushed and sieved by a conventional method to obtain 20-40 meshes) is loaded into a lower fixed bed epoxidation reactor, and the lower reactor is connected with an upper reactor. The lower reactor was inserted into an electric furnace. Then, the self-cooling circulating water of the upper-stage reactor is opened. Then, starting a hydrogen cylinder and a mass flow controller thereof, and controlling the hydrogen flow to be 170 ml/min; the oxygen cylinder was then opened and the oxygen flow was slowly increased to 8 ml/min using its mass flow controller. Accurately controlling the flow of the hydrogen and the oxygen and ensuring that the hydrogen and the oxygen are uniformly mixed before entering the upper-stage reactor. The propylene feed to the lower reactor was then opened and the propylene flow was controlled at 18 ml/min by its mass flow controller. And starting a plasma power supply of the upper-stage reactor to perform dielectric barrier discharge when the three paths of gas flow are stable and the cooling water flow of the upper-stage reactor is stable. Thus, the hydrogen peroxide gas synthesized by the upper-stage discharge enters the lower-stage epoxidation reactor from the air hole between the two sections of reactors under the carrying of the surplus hydrogen, is fully mixed with the propylene gas entering the section of reactor from the lateral line, and enters the TS-1 molecular sieve catalyst bed layer together for epoxidation reaction, and the actual molar ratio of the propylene to the hydrogen peroxide is calculated to be about 5: 1. The reaction temperature of the lower reactor was controlled at 130 ℃ by an electric heating furnace. Analysis by on-line gas chromatography (DB-Wax column (30m × 0.32mm, PEG20M) after 30 minutes of discharge (temperature programmed 50 ℃ for 5 minutes, 10 ℃ to 180 ℃ per minute, 2 minutes for 2 minutes, 20 ℃ to 200 ℃ per minute for 5 minutes) and calculated to give a propylene conversion of 14.8%, a PO selectivity of 94.3%, and an effective hydrogen peroxide utilization of 74.0%.

Comparative example 1 this example illustrates that unmodified large particle size micron TS-1 molecular sieves have poor activity and selectivity for the vapor phase epoxidation of propylene and hydrogen peroxide, with low hydrogen peroxide utilization efficiency:

example 1 was repeated, but the large-grain, micron TS-1 molecular sieve synthesized in the first step did not undergo subsequent TPA-containing⁺When the sodium hydroxide solution is hydrothermally modified and directly used for the evaluation of the gas phase epoxidation reaction of propylene and hydrogen peroxide, the conversion rate of propylene is 4.3 percent, the selectivity of PO is 58.1 percent, and H is₂O₂The utilization rate was 21.5%.

Comparative example 2 this example illustrates that the modified molecular sieve obtained by treating large particle size micron TS-1 according to the sodium exchange method provided in publication J.Catal.,1995,151,77-86 does not catalyze the epoxidation of propylene and hydrogen peroxide in the gaseous phase.

Example 1 was repeated, but the large-grain, micron TS-1 molecular sieve synthesized in the first step was modified not by the hydrothermal modification with sodium hydroxide solution as provided by the present invention, but by the sodium exchange method as provided in publications J.Catal.,1995,151, 77-86. The specific method comprises the following steps: a1 mol/L NaOH solution was prepared, and then 1g of a molecular sieve precursor was added to 100mL of the 1mol/L NaOH solution, followed by stirring at 25 ℃ for 24 hours. Then, the mixture is filtered by suction, dried for 12 hours at the temperature of 110 ℃ and roasted for 6 hours at the temperature of 540 ℃.

The molar ratio of silicon to titanium of the sodium exchange catalyst measured by XFR drops to 29.5 and the molar ratio of sodium to titanium reaches 1.39. The infrared characteristic absorption peak of the framework titanium is measured by infrared spectroscopy and appears at 985cm^-1In the vicinity (FIG. 1C), the intensity is obviously reduced to form a shoulder peak, which is consistent with the literature report. Compared with the analysis result of the parent body, the mole ratio of silicon and titanium of the modified product is greatly reduced, which indicates that the modification of the parent body of the TS-1 molecular sieve by the sodium exchange method reported in the literature is not controllable modification but excessive silicon dissolution. Although a large amount of sodium ions are also present in the sodium exchange molecular sieve, the sodium ions are combined with silicon hydroxyl in a balanced cation form and cause the position of a characteristic absorption peak of framework titanium to be 960cm^-1(mother) moved in the direction of high wavenumber, but 985cm^-1It is as much as 7 wavenumbers higher than the modified molecular sieve of example 1. It can be concluded that the molecular sieve obtained by the process of the present invention in example 1 is substantially different from the modified molecular sieve obtained by the sodium exchange process in this example.

The evaluation of the gas phase epoxidation shows that the modified molecular sieve prepared by the sodium exchange method reported in the literature in this example has a propylene conversion of only 3.0%, a PO selectivity of 83.2%, and H₂O₂The utilization rate is only 15.0%. That is, the catalyst obtained by the sodium exchange method is inferior in catalytic performance (except selectivity) to the gas phase epoxidation reaction of propylene and hydrogen peroxide as a precursor, and the self-decomposition reaction of hydrogen peroxide has high activity, so that the effective utilization rate of hydrogen peroxide is only 15.0%.

Comparative example 3 this example serves to illustrate, on the contrary, that sodium ion retention in the modified molecular sieve is critical when modifying large crystallite micron TS-1 according to the controlled hydrothermal treatment method provided by the present invention.

Example 1 was repeated, but immediately after the fourth step was completed, the modified titanium silicalite molecular sieves were subjected to two repeated conventional ammonium exchange treatments with 0.4M ammonium nitrate at room temperature for 2 hours each. Engineers skilled in the art can prepare the hydrogen form of the catalyst by ammonium exchange on a silicoaluminophosphate molecular sieve according to any published literature reportThe reagent method is used for completing the ammonium exchange work. And after ammonium exchange, carrying out suction filtration on the separated solution by using a Buchner funnel to obtain a molecular sieve filter cake. The filter cake was then dried overnight in an electric oven at 110 ℃ to ensure that the dry content of the solid powder (solids content measured by baking at 500 ℃ for 3 hours) was not less than 90%. And finally, roasting the dried solid powder at the constant temperature of 540 ℃ for 6 hours to obtain the ammonium exchange molecular sieve product. The ammonium exchanged molecular sieve product was then used for test characterization and for propylene vapor phase epoxidation. The ammonium exchanged molecular sieve product had a silicon to titanium ratio of 32.1 and a sodium to titanium ratio of 0.12 as measured by XRF. The infrared spectrum characterization shows that the absorption peak of the framework titanium vibration characteristic of the ammonium exchange molecular sieve product is 963cm^-1(FIG. 3). The reaction results show that the conversion rate of propylene of the ammonium exchange molecular sieve product is 6.3%, the PO selectivity is 52.4%, and the effective utilization rate of hydrogen peroxide is 31.5%.

This example illustrates that the mole ratio of sodium to titanium of the modified molecular sieve obtained in example 1 after conventional ammonium exchange is reduced to 0.12, and the absorption peak of the framework titanium with the infrared vibration characteristics is from 978cm^-1(high sodium state of example 1) Return to 960cm^-1Nearby. Comparing this example with example 1, it can be seen that the removal of sodium ions also results in a substantial reduction in the conversion of the gas phase epoxidation reaction and the effective utilization of hydrogen peroxide. This fully illustrates that the existence of enough sodium ions in the modified TS-1 molecular sieve is the key to the good modification effect of the invention. It can also be seen from the comparison of propylene oxide selectivity that the controlled inorganic alkaline hydrothermal treatment method provided by the present invention may generate a portion of acid sites in the molecular sieve due to the effect of dissolving silicon. The presence of sodium ions simultaneously neutralizes this portion of the acid sites, thus achieving high selectivity approaching 94.3% for the modified molecular sieve of example 1. However, in this case, since ammonium exchange removes most of the sodium ions, the acid sites formed by the modification are exposed, resulting in a very low, yet less selective ammonium exchanged molecular sieve than the parent propylene oxide.

Comparative example 4 this example further illustrates that when large grain micron TS-1 molecular sieves are modified according to the controlled hydrothermal treatment provided by the present invention, it is important to retain sufficient sodium ions in the modified molecular sieve.

Comparative example 3 was repeated, but after obtaining the ammonium exchange catalyst, it was subjected to a further back exchange treatment with a sodium nitrate solution at room temperature for 2 hours. The reverse exchange of the sodium nitrate solution is a conventional ion exchange treatment which is substantially the same as the ammonium exchange in comparative example 3 except that the ammonium salt solution is changed to a sodium nitrate solution. Engineers skilled in the art can do this by ion exchange of zeolitic molecular sieves according to any published literature. After the ion exchange of the sodium nitrate solution (0.15M) was completed, the separation, drying and baking operations after the ammonium exchange were repeated. The obtained sodium nitrate exchange catalyst was used for the test performance and the gas phase epoxidation of propylene.

The sodium to titanium ratio of the sodium nitrate exchanged molecular sieve product was measured to rise to 0.25 by XRF. The reaction results showed that the conversion of propylene in the sodium nitrate exchanged molecular sieve product was 8.7%, the PO selectivity was 85.7% and the effective utilization of hydrogen peroxide was 43.5%.

The above results can further show that when the TS-1 molecular sieve is modified according to the controlled hydrothermal treatment method of alkali metal hydroxide solution provided by the invention, enough sodium ions are important to be retained in the modified catalyst. Meanwhile, the example also shows that for the ammonium-exchanged alkali metal ion modified molecular sieve, the lost alkali metal ion can be recovered to a certain extent through the reverse exchange of the alkali metal ion, so that the gas-phase epoxidation activity, selectivity and effective utilization rate of hydrogen peroxide of the molecular sieve are recovered to a certain extent.

Comparative example 5 this example illustrates that TPA, if used, is absent⁺The alkali metal hydroxide solution carries out controlled hydrothermal modification on the TS-1 molecular sieve, and can also achieve the similar effect of improving the gas-phase epoxidation performance of the TS-1 molecular sieve, but the modified molecular sieve has low yield, the concentration of soluble silicate species contained in the separated mother liquor is high, and the soluble silicate species are difficult to treat and discharge after reaching the standard.

Example 1 was repeated, but when the second operation was carried out, TPA-free was formulated directly⁺0.1mol/L of a hydrogen hydroxideThe sodium solution was used for subsequent modification. Then, the molar ratio of sodium to titanium of the obtained modified molecular sieve is 0.85 measured by XRF, and the infrared characteristic absorption peak position of the framework titanium active center measured by an infrared spectrogram method is 968cm^-1The result of the epoxidation reaction of propylene and hydrogen peroxide is: the conversion of propylene was 15.0; PO selectivity was 96.8%; the effective utilization rate of the hydrogen peroxide is 75.0 percent.

Impressively, the yield of modified molecular sieve (dry weight of modified molecular sieve as a percentage of dry weight of the TS-1 precursor) in this example was only 85%. Meanwhile, the inorganic ash (silicate) in the separated mother liquor of the hydrothermal modification is measured to reach 654.4mg/mL by a burning weight reduction method. In contrast, the yield of the modified molecular sieve increased to 91% when the modification was performed exactly as in example 1. Accordingly, the inorganic ash (silicate) in the separated mother liquor of the hydrothermal modification was reduced to 211.7mg/mL as measured by a weight loss on ignition method.

The above measurement results show that, in the present invention, a small amount of TPA⁺The use of ions not only reduces the modification loss of the titanium-silicon molecular sieve, but also greatly reduces the content of silicate dissolved matters in the alkali metal hydroxide waste liquid generated after modification, so that the discharged waste water is easier to treat and reaches the discharge standard, and the implementation of the modification method of the invention is relatively easy.

Example 2 this example illustrates the invention of introducing a small amount of TPA to an aqueous alkali metal hydroxide solution⁺Quaternary ammonium cation makes the hydrothermal modifying method suitable for the nanometer TS-1 molecular sieve mother body synthesized in the classical method.

Example 1 was repeated, but in the first step of hydrothermal synthesis of the TS-1 molecular sieve precursor, the TS-1 molecular sieve precursor was synthesized according to the classical method formulation described in Chinese patent application No. 200910131993.5. The mole ratio of silicon to titanium, the index data of framework titanium and the index of relative crystallinity of the matrix all meet the requirements of the invention, but the grain size of the matrix is 200-300 nanometers, and the matrix belongs to the commonly known nanometer TS-1. XRF is used for determining that the sodium content of the parent substance of the nano TS-1 molecular sieve is lower than the detection limit, and the sodium-titanium molar ratio of the modified product is 0.49; characterization of skeleton titanium vibration characteristic absorption of nanometer TS-1 molecular sieve matrix by infrared spectroscopyPeak located at 960cm^-1The position of the absorption peak of the vibration characteristic of framework titanium of the modified molecular sieve product is 975cm^-1. The modified molecular sieve product obtains the following results in the propylene and hydrogen peroxide gas phase epoxidation reaction experiment: the conversion rate of propylene of the nano TS-1 matrix is 7.3%, the PO selectivity is 76.7%, and the effective utilization rate of hydrogen peroxide is 36.5%; the propylene conversion rate of the modified nano TS-1 catalyst is 12.3%, the PO selectivity is 94.6%, and the effective utilization rate of hydrogen peroxide is 61.5%.

Comparative example 6 this example illustrates that if the aqueous alkali metal hydroxide solution does not contain TPA⁺The quaternary ammonium cation is obtained by carrying out hydrothermal modification on the nanometer TS-1 molecular sieve matrix by using a simple alkali metal hydroxide aqueous solution, so that the aim of improving the gas-phase epoxidation performance of the nanometer TS-1 molecular sieve is not achieved.

Example 2 was repeated, but the modification solutions prepared in the second step contained only 0.1mol/L sodium hydroxide and 0.01mol/L sodium hydroxide, respectively. Then, the results of the modified catalyst in the experiments of the propylene and hydrogen peroxide gas phase epoxidation reaction are respectively as follows: the propylene conversion rate of the modified catalyst obtained by hydrothermally treating the nanometer TS-1 molecular sieve matrix with 0.1mol/L sodium hydroxide solution is 0.50%, the PO selectivity is 87.3%, and the effective utilization rate of hydrogen peroxide is 2.5%; the propylene conversion rate of the modified nano TS-1 catalyst obtained by hydrothermally treating the nano TS-1 matrix with 0.01mol/L sodium hydroxide solution is 6.9 percent, the PO selectivity is 96.6 percent, and the effective utilization rate of hydrogen peroxide is 34.5 percent;

as can be seen from this example and example 2, if the modifying solution does not contain a small amount of TPA⁺Quaternary ammonium cations, then the propylene conversion and hydrogen peroxide selectivity of the modified molecular sieve product does not exceed that of the parent even when a particularly low concentration alkali metal hydroxide solution is used (in which case the selectivity of the modified catalyst is better than that of the parent, due to acidity neutralization, as previously described). It is apparent that the present invention is achieved by introducing a small amount of TPA into the alkali metal hydroxide modification solution⁺The quaternary ammonium cation broadens the applicability of the modification method to molecular sieve precursors.

Example 3 this example illustrates the modification of TPA in a solution by varying the amount of TPA⁺In an amount ofTo adjust the yield of the modified catalyst. But TPA⁺Cannot improve the catalytic performance of the TS-1 molecular sieve per se.

Example 1 was repeated, but in the second operation, TPA in the solution was modified separately⁺The concentrations were changed to 0.08mol/L and 0.23 mol/L. The yields of the modified catalyst were 90.6% and 91.1%, respectively; the result of the gas phase epoxidation reaction of the obtained modified product on propylene is as follows: propylene conversion was 14.2% and 14.0%, respectively; PO selectivity was 95.4% and 94.6%, respectively; the effective utilization rates of the hydrogen peroxide are 71.0 percent and 70.0 percent respectively.

Comparing the results of this example with example 1, TPA can be seen⁺The quaternary ammonium cation itself does not play a major role in modifying catalysis, its primary role is to affect recrystallization and catalyst recovery.

Example 4 this example illustrates that by varying the modification time parameter, the degree of hydrothermal modification of the alkali metal hydroxide can be controlled and the catalytic performance of the modified molecular sieve for the phase epoxidation of propylene and hydrogen peroxide can be varied.

Example 1 was repeated, but the duration of hydrothermal modification was changed to 2,4, 8 and 24 hours in this order when the third step was carried out. Then, the relative crystallinity data (figure 2) of the resulting modified molecular sieve is 85.0%, 75.8%, 85.7% and 83.4% in that order; the silicon-titanium molar ratio data are 33.3, 33.4 and 33.3 in sequence; the molar ratio data of sodium to titanium are 0.45, 0.53, 0.62 and 0.57 in sequence; the positions of infrared characteristic absorption peaks (figure 3) of the framework titanium active center are 973, 976 and 978 in sequence. The result of the above modified molecular sieve on the epoxidation reaction of propylene and hydrogen peroxide gas phase is as follows: the propylene conversion rates were 8.5%, 9.5%, 12.7% and 9.4% in this order; the PO selectivity was 92.4%, 92.5%, 93.5% and 95.0% in this order; the effective utilization rates of the hydrogen peroxide are 42.5%, 47.5%, 63.5% and 47.0% in sequence. As mentioned above, the hydrothermal treatment time used in example 1 was 16 hours, and the propylene conversion rate of the obtained modified molecular sieve was 14.8%, the PO selectivity was 94.3%, and the effective utilization rate of hydrogen peroxide was 74.0%. This indicates that the hydrothermal treatment time is within a suitable range. Therefore, the invention provides the preferable range of 10-20 hours, and the more preferable range of 15-20 hours.

However, from the comparison with the parent reaction results (comparative example 1), it can be seen that the effectiveness of the modification method provided by the present invention for modifying the TS-1 parent can be demonstrated over a wide time frame.

Example 5 this example illustrates that the degree of hydrothermal modification can also be controlled by changing the alkali metal hydroxide concentration parameter of the modification solution, and the catalytic performance of the modified molecular sieve on the propylene and hydrogen peroxide gas phase epoxidation reaction is changed accordingly.

Example 1 was repeated, but in the second step of the operation, the sodium hydroxide concentration in the prepared modified solution was changed to 0.05, 0.15 and 0.20 in this order. Then, the infrared characteristic absorption peak positions of the framework titanium active center of the modified molecular sieve measured by an infrared spectrogram method (attached figure 4) are 973, 978 and a weak shoulder peak in sequence, and the result of the epoxidation reaction of propylene and hydrogen peroxide gas phase is as follows: the conversion rate of propylene is 10.3 percent, 11.7 percent and 8.4 percent in sequence; PO selectivity was 82.3%, 94.4% and 90.5% in this order; the effective utilization rates of the hydrogen peroxide are 51.5 percent, 58.5 percent and 42.0 percent in sequence. Considering that the alkali metal hydroxide concentration used in example 1 was 0.1mol/l, the propylene conversion, PO selectivity, and hydrogen peroxide effective utilization of the resulting modified molecular sieve were 14.8%, 94.3%, and 74.0%, respectively. It can be seen that there is also a suitable range of alkali metal hydroxide solution concentrations. Therefore, the invention provides that the preferable range is 0.01 to 0.20mol/L, and the more preferable range is 0.05 to 0.15 mol/L.

Also, the present invention is intended to demonstrate that the effectiveness of the modification process provided by the present invention for modifying precursors of TS-1 molecular sieves can be demonstrated over a wide range of alkali metal hydroxide concentrations, as can be seen from comparison of the reaction results with the precursors (comparative example 1).

Example 6 this example illustrates that by varying the temperature parameters, the degree of hydrothermal modification of the alkali metal hydroxide can also be controlled, and the catalytic performance of the modified molecular sieve for the epoxidation of propylene with hydrogen peroxide gas changes accordingly.

Example 1 was repeated, but in the third step, the temperatures for modification by hydrothermal treatment were changed to 25, 80, 110 ℃, 150 ℃, 190 ℃, 210 ℃ in this order. The result of the epoxidation reaction of propylene and hydrogen peroxide gas phase of the obtained catalyst is then as follows: the conversion rates of propylene are 3.9%, 6.0%, 10.4%, 12.8%, 11.9% and 7.1% in sequence; the PO selectivity is 92.0 percent, 92.3 percent, 93.2 percent, 92.4 percent and 92.3 percent in sequence; the effective utilization rates of hydrogen peroxide are 19.5%, 30.0%, 52%, 64.0%, 59.5% and 35.5% in sequence. Considering that the hydrothermal treatment temperature used in example 1 was 170 ℃, the propylene conversion, PO selectivity, and hydrogen peroxide effective utilization of the resulting modified molecular sieve were 14.8%, 94.3%, and 74.0%, respectively. This indicates that the hydrothermal treatment temperature also has a suitable range. Therefore, the optimal range of the invention is 100-200 ℃, and the more optimal range is 150-190 ℃.

Here, the invention is to claim, from the precursor reaction results (comparison of example 1) can be seen, the modification method of the invention for TS-1 precursor modification effectiveness can be achieved in a wide range of hydrothermal treatment temperature.

Example 7 this example illustrates that by adjusting the liquid-solid ratio parameters, the degree of hydrothermal modification of alkali metal hydroxide can be adjusted and controlled, and the catalytic performance of the modified molecular sieve for the phase epoxidation reaction of propylene and hydrogen peroxide can be changed accordingly.

Example 1 was repeated, but in the third step, the liquid-solid ratios modified by the hydrothermal treatment were changed to 4, 5, 7 and 15 in this order. The modified molecular sieve has the following results for the hydrogen peroxide gas phase epoxidation of propylene: the propylene conversion rates were 9.4%, 11.8%, 13.7% and 9.8% in this order; PO selectivity was 93.0%, 94.5%, 93.7% and 93.1% in this order; the effective utilization rates of hydrogen peroxide are 47.0%, 59.0%, 68.5% and 49.0% in sequence. Likewise, considering the liquid-solid ratio of 10 employed in example 1, the propylene conversion, PO selectivity and hydrogen peroxide effective utilization of the resulting catalyst were 14.8%, 94.3% and 74.0%, respectively. Obviously, there is also a suitable region for the liquid-solid ratio. Therefore, the invention provides the preferable range of 5-15, and the more preferable range of 8-12.

Therefore, from the comparison with the parent reaction results (comparative example 1), it can be seen that the effectiveness of the modification method provided by the present invention for the modification of the TS-1 parent can be demonstrated in a wide range of liquid-to-solid ratios.

Example 8 this example illustrates that the use of an appropriate low concentration alkali metal hydroxide solution as a wash solution in the washing step after hydrothermal treatment is beneficial to achieving a modification effect.

Example 1 was repeated, but in the fourth step of the work-up washing step the filter cake was washed successively with deionized water, 0.001, 0.005 and 0.05 mol/l sodium hydroxide solution. When the filtrate is washed until no precipitate is generated after neutralization, the sodium-titanium molar ratio data of the obtained catalyst are 0.20, 0.45, 0.46 and 0.48 in sequence. The results of the above-described catalyst for the vapor phase epoxidation of propylene and hydrogen peroxide were as follows: the conversion rates of propylene are 8.8%, 14.2%, 14.8% and 14.6% in sequence; PO selectivity is 85.1%, 93.9%, 94.2% and 94.5% in sequence; the effective utilization rates of the hydrogen peroxide are 44.0%, 71.0%, 74.0% and 73.0% in sequence.

Example 9 this example illustrates that potassium hydroxide is equally effective in accordance with the controlled hydrothermal treatment provided by the present invention.

Example 1 was repeated, but in the second step the hydrothermal modification solution was prepared, potassium hydroxide was used instead of sodium hydroxide. Then XRF analysis is carried out to obtain a modified sample with the silicon-titanium molar ratio of 33.4 and the potassium-titanium molar ratio of 0.38; the result of the phase epoxidation reaction of propylene and hydrogen peroxide by the potassium ion modified TS-1 molecular sieve is as follows: propylene conversion 13.8%, PO selectivity 94.6% and hydrogen peroxide effective utilization 69.0%.

Example 10 this example illustrates that lithium hydroxide is equally effective when modified according to the controlled hydrothermal treatment of alkali metal hydroxide provided by the present invention.

Example 1 was repeated, but in the second step of preparing the hydrothermal modification solution, lithium hydroxide was used instead of sodium hydroxide. Then, the result of the lithium ion modified TS-1 molecular sieve on the propylene and hydrogen peroxide gas phase epoxidation reaction is: propylene conversion 13.5%, PO selectivity 94.2% and hydrogen peroxide effective utilization 67.5%.

Comparative example 7 this example illustrates that the modified TS-1 molecular sieve obtained according to the process of the present invention has an improved effect on the vapor phase epoxidation of propene and hydrogen peroxide, but has no significant improvement effect on the liquid phase epoxidation of propene and hydrogen peroxide.

The liquid phase epoxidation reaction may be carried out according to any of the methods described in the publications. Specifically, in this example, the liquid phase epoxidation reaction was carried out in a 450ml stainless steel reaction vessel, temperature controlled by a water bath, and magnetic stirring. The experimental conditions were as follows: the reaction temperature was 40 ℃, the propylene pressure was 0.6MPa, and the reaction time was l h. The ingredients are as follows: the catalyst amount was 0.2g, methanol 30ml, H2O2 (30%) 2 ml. Before the experiment, the autoclave was pressurized with propylene gas and then vented. The replacement is repeated for 5-6 times in order to replace the air in the reactor. Measuring H by iodometry₂O₂The organic composition was analyzed by chromatography.

TABLE 1 accompanying Table 1 data for liquid phase epoxidation of comparative example 7 (using catalysts from examples 1 and 4)

In this example, liquid phase epoxidation reactions were carried out using the modified molecular sieves of example 1 and example 4, respectively. The results are detailed in attached Table 1. As can be seen from the attached Table 1, if the modified molecular sieve prepared by the method of the present invention is used in the liquid phase epoxidation reaction, the conversion rate of the raw material hydrogen peroxide is reduced, and the effective utilization rate of the hydrogen peroxide is also reduced. The improved selectivity of the modified molecular sieve in the liquid phase epoxidation reaction is actually the result of sodium ions neutralizing a small amount of weakly acidic side reaction centers on the catalyst surface. These are in accordance with the results obtained in J.Catal.,1995,151,77-86 on sodium exchanged TS-1 molecular sieves. The important information to be emphasized in this example is: by the process of the invention-containing a small amount of TPA⁺Alkali metal ion modified framework titanium obtained by alkali metal hydroxide solution controlled hydrothermal modification method of quaternary ammonium cationThe active center is also not beneficial to the liquid phase oxidation reaction. The presence of sodium ions on the silicon hydroxyls near the framework titanium hinders the liquid phase oxidation reaction (reducing conversion) but is relatively favorable for the self-decomposition reaction of hydrogen peroxide (reducing the effective utilization rate). The experimental results prove that the alkali metal ion modified framework titanium active center is beneficial to gas-phase epoxidation reaction, and is an important finding per se.

Claims (9)

1. The alkali metal ion modified titanium silicalite molecular sieve TS-1 for the hydrogen peroxide gas phase epoxidation reaction is characterized in that in the alkali metal ion modified titanium silicalite molecular sieve TS-1, alkali metal ions are retained on silicon hydroxyl groups of the modified TS-1 molecular sieve, and the infrared characteristic absorption band of an alkali metal ion modified framework titanium active center is higher than 960cm^-1And less than 980cm^-1Within the range of (1); the TS-1 molecular sieve parent body of the alkali metal ion modified titanium-silicon molecular sieve TS-1 meets the following requirements: the molar ratio of silicon to titanium is less than or equal to 200; the index value of the framework titanium content is more than or equal to 0.40; the relative crystallinity is more than or equal to 85 percent;

the preparation method of the alkali metal ion modified titanium silicalite TS-1 adopts TPA containing tetrapropyl quaternary ammonium cation⁺The alkali metal hydroxide modified solution carries out controlled hydrothermal treatment on a TS-1 molecular sieve parent, and the method comprises the following specific steps:

in the first step, a composition containing TPA is prepared⁺Ionic alkali metal hydroxide modification solution

In the modified solution, the concentration of the alkali metal hydroxide is 0.01-0.20 mol/L; TPA⁺The ion concentration is 0.05-0.50 mol/L;

in the second step, with a composition comprising TPA⁺Controlled hydrothermal treatment of TS-1 molecular sieve precursors with ionic alkali metal hydroxide modification solutions containing TPA⁺The ratio of the dosage of the ionic alkali metal hydroxide modified solution to the dosage of the TS-1 molecular sieve parent material is 5-15 ml/g molecular sieve; the hydrothermal modification temperature is 100-200 ℃; the hydrothermal modification time is 10-20 hours;

thirdly, post-treatment of the hydrothermally modified TS-1 molecular sieve

The post-treatment comprises solid-liquid separation, washing, drying and roasting; in the washing process, washing the modified TS-1 molecular sieve wet material obtained by solid-liquid separation by using a low-concentration alkali metal hydroxide solution, wherein the washing degree is based on that no precipitate appears after the washing solution is neutralized by acid and alkali; the concentration of the alkali metal hydroxide solution is 0.001 to 0.05 mol/L.

2. The alkali metal ion modified titanium silicalite TS-1 for the vapor-phase epoxidation of propylene with hydrogen peroxide according to claim 1, wherein the molar ratio of silicon to titanium of the parent molecular sieve TS-1 is not more than 100; the index value of the framework titanium content is more than or equal to 0.45; the relative crystallinity is more than or equal to 90 percent.

3. The alkali metal ion modified titanium silicalite TS-1 for the hydrogen peroxide gas-phase epoxidation reaction of propylene according to claim 1, wherein in the first step, the concentration of alkali metal hydroxide in the modified solution is 0.05-0.15 mol/L; TPA⁺The ion concentration is 0.10-0.30 mol/L; the alkali metal hydroxide is lithium hydroxide, sodium hydroxide or potassium hydroxide; providing TPA⁺The ionic compound is one or more of tetrapropylammonium chloride, tetrapropylammonium bromide, tetrapropylammonium fluoride and tetrapropylammonium iodide.

4. The alkali metal ion-modified titanium silicalite TS-1 of claim 1, wherein in the second step, comprises TPA⁺The ratio of the dosage of the ionic alkali metal hydroxide modified solution to the dosage of the TS-1 molecular sieve parent material is 8-12 ml/g molecular sieve; the hydrothermal modification temperature is 150-190 ℃; the hydrothermal modification time is 15-20 hours.

5. The alkali metal ion-modified titanium silicalite TS-1 of claim 3, comprising TPA in the second step⁺The proportion of the dosage of the ionic alkali metal hydroxide modified solution to the dosage of the TS-1 molecular sieve parent material is 8-12 mlPer gram of molecular sieve; the hydrothermal modification temperature is 150-190 ℃; the hydrothermal modification time is 15-20 hours.

6. The alkali metal ion modified titanium silicalite TS-1 for the vapor-phase epoxidation reaction of propylene and hydrogen peroxide according to claim 1, wherein in the third step, the concentration of the alkali metal hydroxide solution used for washing is 0.005-0.04 mol/L; the alkali metal hydroxide is lithium hydroxide, sodium hydroxide or potassium hydroxide; the drying temperature is 80-120 ℃, and the drying time is selected according to the dry basis content of the sample not less than 90%; the final roasting temperature is 400-550 ℃, and the constant temperature time at the final roasting temperature is more than 3 hours.

7. The alkali metal ion modified titanium silicalite molecular sieve TS-1 for the vapor-phase epoxidation reaction of propylene and hydrogen peroxide according to claim 3,4 or 5, wherein in the third step, the concentration of the alkali metal hydroxide solution used for washing is 0.005-0.04 mol/L; the alkali metal hydroxide is lithium hydroxide, sodium hydroxide or potassium hydroxide; the drying temperature is 80-120 ℃, and the drying time is selected according to the dry basis content of the sample not less than 90%; the final roasting temperature is 400-550 ℃, and the constant temperature time at the final roasting temperature is more than 3 hours.

8. The alkali metal ion modified titanium silicalite TS-1 for the hydrogen peroxide gas-phase epoxidation reaction of propylene according to claim 6, wherein in the third step, the concentration of the alkali metal hydroxide solution used for washing is 0.005-0.03 mol/L.

9. The alkali metal ion modified titanium silicalite TS-1 for the hydrogen peroxide gas-phase epoxidation reaction of propylene according to claim 7, wherein in the third step, the alkali metal hydroxide solution used for washing has a concentration of 0.005-0.03 mol/L.

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